Just leaving a weeklong shooting trip to California and Utah with my Z7 (the images are on the computer, but I haven't even seen them on the big monitor yet, let alone made prints) - writing on the flight back... My initial impressions are confirmed by this extensive use in a very concentrated period of time (1000 images in 8 days). Nikon has quite the camera here - while I agree with Thom Hogan's impressions that, in most ways, it's what Sony would call a Mark 2.5 or Mark 2.6 (it doesn't quite have the focus system or battery life of a Mark III Sony, and it's a long way from the lens selection unless you count the FTZ adapter), it's notably superior to even a Mark III in a few ways that happen to matter for my particular kind of shooting.
1.) It's really well built and sealed - both Roger Cicala at Lensrentals and Imaging Resource's water test show the Z7 in a class with only a couple other mirrorless cameras (the Fuji X-H1 is similar, with the Olympus E-M1 mk II and presumably the E-M1x even better). Neither Canon nor Sony has this level of ruggedness in their FF mirrorless lineup, although the Panasonic S1 line might be.
2.) I happen to like a couple of the compact zooms. The 24-70 f4 has served me very well, and I just got a 14-30 in time for this trip. The only Sony 24-70 f4 is the older "Zeiss" lens, which doesn't hold a candle to the Nikkor. Sony does have a 24-105 that seems close, although substantially bulkier - and there's, of course, the G-master - if you're willing to carry a substantially larger lens, I've always heard it to be excellent. The 14-30 is a smallish, sharp lens that takes filters and is wider than Sony's closest equivalent, although it doesn't get as wide as the somewhat bulkier and more expensive Sony 12-24.
3.) ISO 64. The sensor in a modern Sony and a modern Nikon are very similar (both A7III vs Z6 and A7rIII vs. Z7), but Nikon lets you get 2/3 of a stop lower in ISO on the Z7, reducing noise still farther. At ISO 64, noise just isn't there - not that a modern sensor has much at 100 either, but the utterly noiseless, high-DR ISO 64 setting is special.
4.) The Nikon just feels more like a camera to me - it hasa really nice interface, a great grip, etc. Fuji and Nikon, while very different, have gotten the interface to a camera right.
Of course, a Sony A7rIII will track action better (probably even than a Z with the firmware update, although I haven't used the two next to each other). The Sony has a wider selection of lenses, although Nikon is hitting a home run with pretty much every lens they release right now.
As I see the state of the mirrorless industry right now:
Nikon: Their early-generation bodies are excellent, with the Z7 using the incomparable (ISO 64) D850 sensor. They are clearly not quite as refined in some ways as a MK III Sony, but they are a lot closer than a first generation camera might be. The lens lineup isn't where Sonys is, but they are releasing sensible lenses at a good clip. To me, the biggest lack is anything longer than 70 mm - the Z system could really use a small, light 70-300 or similar (how about a 70-300 f4-5.6 PF, Nikon)? Every lens they release is excellent for what it is - they aren't terribly ambitious super-speed designs for the most part, but they are a really nice bunch of sharp, compact lenses.
If I ran Nikon, I'd keep filling out the lens lineup, working on firmware updates, and maybe making a high-speed A9 competitor (is there enough market for both a Z9 and the D6?) and/or a video-centric body. The Z7 probably gets iterated with the D850 - every time one receives a sensor advance, the other does as well. The FTZ adapter is a very stable way of extending the lens lineup. They need to decide how to feed people into the Z system - is it an APS-C Z-mount body or bodies (and lenses), a stripped down FF body, one of each?
Nikon deals with their lens mount variations relatively well. Older autofocus lenses introduced in the film era won't autofocus on many newer bodies, and a few very new (AF-P) lenses, all but one of which are inexpensive APS-C lenses likely to stay with the body they were bought with, don't work on older bodies, but those are the only meaningful incompatibilities. There are other incompatibilities, but they only affect cases where either the body or the lens is at least 30 years old - the most prominent is the case where neither body nor lens has a way of controlling aperture (only relevant to some film bodies - all digital bodies have aperture control). Even absurd combinations like an 18-55mm DX kit lens on a $3000+ Z7 using the FTZ adapter generally work (as does a 600mm f4 on a D3000). Of course that $100 DX kit lens on the Z7 is going to vignette like crazy, but it's a wonder that it will take a picture at all, much less with AF and dual VR working.
Sony (FE): Currently the folks to beat in full-frame, but they've been resting on their laurels. The A7rIII is still a great camera, but it's two years old, and uses a sensor two years older than that. The D850/Z7 sensor is a pretty good refinement of the A7rII and III sensor, mainly because of the lower ISO. An A7r IV might well regain the resolution crown while retaining superb performance in other aspects of image quality. Panasonic has set a high bar for any A7s III's video modes, especially with the S1h coming out.
Sony's lens line is relatively complete - there are a few exotics missing, and some of the early, weaker FE lenses could use updates. Sony could benefit from a tilt/shift lens or two, since the only current option is a third-party adapter and a Canon (or Nikon) lens - Sony body to Kipon, Fotodiox, etc. adapter to Canon lens isn't as reliable as Canon body to Canon adapter to Canon lens (or all Nikon). Oddly, their real need in FE now is bodies, not lenses, and updated lenses rather than expansion of the line.
While Sony doesn't have much in the way of first-party guaranteed adapters like Canon and Nikon do (old Minolta lenses is a far less useful adapter than 30 years of EF, or 60 years of Nikkors) third-party adapters will mount essentially any lens ever produced to a Sony body.
Sony (APS-C): Not much to talk about - we haven't seen a real update in a couple of years, since the a6400 was a very minor update, and the lens situation is terrible, other than FE lenses (why not use an FE body, given the low prices of some older models)? The very capable a6500 is still generally bundled with the 16-50mm powerzoom, which may be the worst interchangeable lens in current production, other than Lensbabies, etc. (despite some other honorable-mention APS-C kit lenses). The old a6000 is extremely inexpensive, and perhaps a worthy mirrorless competitor to the D3500/low-end Rebels, which Fuji really doesn't do - the primarily Asian-market Fuji X-A cameras are hard to find in the US (and Europe?), but every option above that is inferior to a comparably priced Fuji due to lenses and controls. The exception, of course. is as spare bodies for full-frame Sony owners.
Canon (EOS-R): Where are the bodies those gorgeous lenses are supposed to go on? Canon has made some absolutely beautiful (and frightfully heavy/expensive) lenses, and introduced them with bodies that look like they were designed for entirely different lenses. Even the original EOS-R isn't really the right body for a 28-70 f2.0 or an Otus-sized 50mm, and they're laughable on an EOS-RP. The only lenses really made for lower-end bodies are the 24-240 travel zoom and the 35mm (??) macro. The 24-105 is also a decent match for the R, although too expensive for the RP. The next lens out is an 85 mm f1.2 for $2700???
Canon needs at least two more bodies that match the high-end lenses - a very high resolution monster, probably well above 50 MP, and a high-performance pro body (in the 30+ MP range, but with a sensor with excellent dynamic range and AF performance). These need to be seriously weather sealed, highly controllable pro bodies at or above the 5D level. The two current bodies are more in the 6D class, with the RP coming in even below the 6D. Also, an adapted kit lens?? Canon badly needs more lower-end lenses, including something to kit with the RP.
Canon (EOS-M): A decent lower-end system, with some midrange options. Lack of R to M compatibility in either direction is a little puzzling. Canon now has four currently maintained lens mount variations - EF, EF-S, EF-M and EF-R. Full-frame DSLRs (and some cine cameras) only take EF lenses while APS-C DSLRs (and other cine cameras) take EF and EF-S lenses. The mirrorless systems confuse the situation further, as EOS-R cameras take EF-R lenses natively, and EF with an (excellent) adapter, while EOS-M cameras take EF-M and adapt both EF and EF-S (but have no way to mount an EF-R lens).
Fuji: A very well thought out pair of systems, with excellent lenses in both APS-C and small medium format. They have the best overall APS-C lens line on the market, and quite possibly the best line in any format excluding the decades-old Canon and Nikon FF lines. Their medium format lens line is growing rapidly, and is superb. Their bodies are also among the best to use...
Their one real problem is that they are dependent on sensor sizes that they are either the highest-end customer for (everybody uses APS-C, but there are very few APS-C cameras priced like an X-T or X-H camera, or with the performance expectations) or one of the few customers for (I suspect that over 80% of 33x44 mm sensors go to Fuji - Hasselblad and Pentax both use a few). If Sony updates all the sensors together, Fuji has nothing to worry about (their sizes actually make a lot of sense), but if they let APS-C stagnate because everyone except Fuji wants cheaper sensors for $400 cameras instead of better ones for $1500 cameras, or if they decide that updating medium format isn't worth it for Fuji's business, they can leave Fuji with stale sensors.
Right now, the X-T3 sensor is competitive, very close to even a 24 MP full-frame sensor (by many measures, it actually outperforms the Canon 6D sensor despite the size difference). What we've seen of the GFX 100 sensor suggests that it is very, very close to the state of the art - only the Phase One IQ4 150 can beat it, at a huge cost both in money and usability. Fuji's got great sensors right now, but they probably wish they were less at Sony's mercy. One sensor size shared mainly with <$500 cameras (as everyone except Fuji abandons high-end APS-C) and another where they're most of the volume makes things tricky.
Panasonic: They finally released the S camera that makes sense... Their expertise has long been in still/video hybrids, and the S1h looks like the most capable one yet. The S1 and S1r make sense as ecosystem partners for the S1h, but it's the h, and video, that define Panasonic's reason for being. A bigger camera with fewer lenses, expensive lenses and no special capabilities is a tough sell against the A7III and Z6, while the high resolution mode may bring some fans to the S1r, but it's limited enough that many photographers will opt for a more compact A7rIII or Z7 with more lens choices.
Their investment in Micro 43 makes relatively little sense except to support their existing customers. Their most exciting camera is the GH5 and its close relative the GH5S, which are big and expensive for Micro 43, and not all that far off the price of an S1 (especially the GH5S). The G (not GH) series has a hard time making an argument for itself against midrange DSLRs, the X-T30 and X-E3, and even the bottom end of full-frame (D610, EOS-6D, EOS-RP and older model Sonys). A Micro 43 generalist camera (neither video-focused, nor a sports/action camera, nor really tiny) should be cheaper than other generalist cameras with larger sensors.
They also have an issue with video-focused cameras using no less than three lens mounts (the third is the EVA-1, a $6000 6K camera in a more conventional camcorder shape, which uses Canon EF lenses). If the successor to the EVA-1 is L-mount, or has an L-mount version available, that would help.
Olympus: They need to come out with MK III versions of the cameras that make Olympus Olympus - the E-M5 and E-M1. The tiny low-end Pen cameras don't sell much here in the US (they may be Asian-market specials like several low-end Fujis we just don't see). The Pen-F doesn't seem to have gotten much traction against Fuji. On the other hand, there's nothing else with decent weather sealing or close to the speed that is as small and light as an E-M5, and there is nothing with the features of a pro sports camera at anything close to the price or weight of an E-M1.
They really need a new sensor - unfortunately, they don't make those. A 20 MP Exmor RS (stacked) sensor with all the modern tricks would help a lot. With the E-M1 line's focus on performance, a stacked sensor makes a ton of sense here...
The E-M1x was pretty much a diversion - it's really an E-M1 mkII with a grip and some new firmware. A true E-M1 mkIII would feature a new sensor, and it would be closer to the mkII in size and price. It might get some of the firmware from the E-M1x.
Another odd camera Olympus might consider making is a truly Tough mirrorless - something like their Tough compact line, but with interchangeable lenses. Nikon did that with the AW1, but I think Olympus could do a better job... The AW1 was viewfinderless, which is a killer for a camera that is going to be used outdoors almost all the time. The camera I have in mind would be based on either an E-M5 or E-M10, but with oversized controls to be used with gloves on, and sealed enough for snorkeling, shockproof enough for skiing. It would need a modified lens mount that could take either standard Micro 43 lenses (becoming as sealed as the lens - with some of the PRO lenses, that's quite good - although not underwater) or special lenses that maintain the full sealing.
Pentax: Are they going to play in mirrorless at all, or just keep making DSLRs for people with existing Pentax lenses?
Leica:Irrelevantly expensive for most people - I see no sign of real differentiation that would make the price worth it (a SL body and lens can easily run as much as a GFX 100, and it's 24 MP). Even if they come out with a new SL that uses the S1r sensor, how do they differentiate it from the S1r?
Hasselblad: An X1D that uses the GFX 100 sensor could be interesting (although probably too pricey for all but a very few)...
