I shoot m4/3s for my own fun and only changed from 4/3s because Olympus stopped developing that line. Panasonic and Fuji didn't have a large installed user base to worry about, nor really did Olympus, so I sort of understand why they went mirrorless, insofar as it's cheaper to assemble (I'm told).
What I don't quite understand is why "full-frame" Canikon users are changing from D-SLRs to the new mirrorless models. What is the perceived user advantage? If Olympus had continued with their D-SLR 4/3s line, I would probably have stuck with it. I like some things about mirrorless, having the histogram before taking the pic for instance, but I got by without those things before and would not have spent the money changing systems.
To be honest, I think I spoke a little too soon on the mirrorless systems and whether or not it is worth getting. I was thinking from an handheld perspective and that one already owned a relatively recent 35mm DSLR. Personally the bodies of the mirrorless systems are just too small to be comfortable in the hands IMO. Not to mention on a image quality perspective, the increases have been less and less dramatic. If you cant take a good picture with any camera that has been on the market for the last 5 years, it has nothing to do with the camera. So the question is, are there any real advantage to switching over if you don't need to? If you already own a good 35mm, only ever shoot handheld and have some money to burn, I would suggest buy stocks instead.
But I think the real advantage of the mirrorless systems are the modularity of them, especially the Sony, just like with a digital back.
When I started out 12 years ago, if you needed tilt/swing and shift, you pretty much had two options, the Canon system with the t/s lenses or a digital back on a technical camera. The Canon system was an okay option, but not great by any stretch. At the time, they only had one good t/s lens, the 24mm, and the IQ of the files were not great, but the system was only about $6k or so. The digital back on a tech camera was a much better option, better lenses and more of them, much better IQ, more movements and tilt/swing was great too. But the systems are around $40K, so very prohibitive for most people who are not either rich or a professional that can secure a loan. Today, the Canon lenses are still far behind the technical camera lenses, so the new mirrorless now provides a third option that gets you almost to the quality of DB system. Since there is no mirror box, you can use a mirrorless camera on a bellow camera, allowing for the full advantages of tilt/swing and shift, but without the $40K price tag. Sure the bellows camera will add on another $4K to $5K, but that is still pretty reasonable for most pros and advanced hobbyists. Additionally, many schools dropped the large format courses since film was getting to be too much of a hassle and buying multiple digital backs and bellow cameras was too expensive (many due to the backs). But now I can see the mirrorless bringing that back.
Another advantage is that you can pretty much use any lens ever made on these cameras, so long as you can get the right mount. Recently I saw portraits shot with a Schneider 135mm projection lens with a fixed f/2 aperture. Someone had a custom barrel made for the lens so it could be focused and mounted to a camera. The pics were really nice; super sharp where in focus but with almost no depth of field.
So for these options, I see mirrorless being a real great system. But for someone who does the typical handheld hobbyists shots that already owns a camera, I would suggest sticking with your current system until it breaks, then look into mirrorless.
Insofar as being able to see the histogram while you are composing, I would advise against pay any attention to it. Often, if you shoot for a perfect histogram you end up having a boring image. Highlights don't always need to be recoverable, and shadows don't always need to be recorded. Although I think everyone here would agree with this, photographers are tech heads in general. Giving yourself just another means to satisfy your techiness will just get in the way of your creativeness. Kind of the same thing when people review at every image they capture.