If your going to test the A7 with a thought of buying, do yourself a favor and try the em1. Michael has a good write up about it and I agree with most of what he says, especially in usability and build quality...
Good recommendations and pragmatic words of advice, but there is a BUT
BC speaks from the viewpoint of someone who, by his own admission, now concentrates and has moved towards motion as a distinguishing quality of his business - autofocus, tracking, stabilisation, colour, in-camera corrections and extensability are all important. M43 sensor size is little different from the sensors he's using for motion.
For someone who, like myself, does mainly B&W, stills only, prefers manual focus, has a host of top-grade legacy glass (that he's not willing to discard) and who values focus peaking and tethering the way BC values AF speed and tracking there is a 'dilemma'.
For reasons I still can't fathom, m43's don't suit 'adapted' lenses. I haven't tested M-glass on the OM-1 (BC has) but did on the Panasonic GH2. The results weren't even in the same ball park.
I suspect that all non-native lenses, either primes or zooms, will give poor(er) results on bodies that use an AA filter. It's not just ray angle, exit pupils et al. It's probably what the OLPF does. Cameras that are are OLPF free have consistently given better results – there are reports both on this site and elsewhere that QED this**. Doesn't mean that there won't be issues with corner resolution, smearing etc – there may well be. It's a question of degree. I suspect this is the main difference between the m43 OMs , Panasonics et al v the Fuji (and other) AA-less CSC's - without discussing high ISO and file quality.
The problem investing today in m43, is twofold. The lenses are good, very good. They're also expensive. But you're buying and committing to that format for the foreseeable future. The lenses are not adaptable, at least not to APS-C and FF. That's an expensive gamble - 35mm (ff) lenses and up, with the arrival of the mirrorless, game changing CSC's - potentially are.
(note to BC : would you be as keen on a Leica S2 if your legacy glass wasn't adaptable to it ?)
I have tested Leica M-glass on the Fuji x-trans, and the results were stellar. I've also used an 85/1.4 with the Metabones SpeedBooster for a quick test. With focus peaking, I nailed focus on the eyes from 6ft away in an instant - there was minimal dof. I've never done that before.
Regarding build quality, there is a big difference between the A7 and the A7r. File quality - I suspect the 'r' version is similar to the D800e, given that they have the same sensor, although different electronics and despite Sony's suspect RAW compression.
I don't know if the OM has focus peaking, but I tried the SONY for a few seconds and it's without doubt the best focus peaking implementation I've yet seen. Enough to persuade me to order the camera, virtually unseen.
Yes, the shutter shake issue is a pain. But to put it in perspective, it's still less than the D800 and I hope that just like Canon, with the 5D's, the initial problems will be resolved with a firmware fix.
So in favour of the SONY, I would suggest that anyone who values 36MP, IQ, focus peaking, (tethering) and future interoperability – there's still a decision to be made.
(And no, I haven't made it yet - the box is still unopened ..)