My "main" camera is a 5D3. I really enjoy that camera and almost everything about it. I'm even okay with its size and weight, while using it. It's only when traveling (by air) that I don't enjoy it.
I also got the a6000 a little more than a year-and-a-half ago. For me, it fills a niche between my phone camera and my "big stuff".
Generally speaking, my "nicer" lenses have a purpose, a "feel". That lens design and feeling is optimized for a full-frame sensor. Yes, a 135/2 can be mounted on a crop-sensor camera, but it really isn't the same lens. For general purpose photography and a walk-around lens, it's pretty hard to beat the convenience of a phone camera. But the a6000 puts all the manual controls, and focus peaking, and real-time effects in the viewfinder, and (better) responsiveness into a camera that's not on the phone.
I think that full-frame mirrorless is kind of in a weird place. If you're looking for small or light, then full-frame doesn't do that. Why? Because it's all about the lenses. When you want good lenses (i.e., big), then the size and weight of the body quickly becomes irrelevant. If you're after the other features of mirrorless, then full-frame is fine.
So for mirrorless, I start with the lenses. Actually, for ANYTHING photography, I start with the lenses. Then pick a body to match.
Because I have a lot of Canon lenses, I also got the Metabones adapter for mounting my Canon lenses on the Sony body. Actually, I got both the "normal" adapter and the "Speed Booster". Autofocus DOES work with many of my lenses on the Sony. It's definitely slower than when using a Canon body, and sometimes a lot slower. But it often works. Some lenses work better with the "normal" adapter, and others work better with the "Speed Booster". It's really the kind of thing that you have to try each particular lens.
There are cheaper adapters, and I've borrowed and used a few, but I've had the best luck with Metabones. There have been improvements and new options that have appeared in the past year or so, so this may have changed.
One thing that I really like with the Sony is the frame rate. I shoot high-school sports for a couple of schools and for various sports. I actually don't use auto-focus when shooting some sports (and when using the Sony) because the focus-peaking is that good. I can do this for tennis and swimming. For football, soccer and lacrosse, I'll go with AF and use the Canon. But, when Sony fits and with 11 frames per second, I have a "sports body" that's a fraction of the cost of any mirrored option.
What I like (about the Sony):
- Small, lightweight for traveling when using small lenses.
- The sensor is good and high resolution.
- "What you see is what you get" in the viewfinder.
- Having a viewfinder (instead of "only" the live view monitor).
What I don't like:
- Already been said: the menus. Non-intuitive.
- Customization and custom modes. Not like Canon (and I like Canon). I want to be able to select C1, C2, etc. But the button assignments don't follow the modes. So *some* settings stick with the C1, C2, etc, but not all. For how I use that feature, it's "broken" to me.
- Control knobs are a little bit cramped, but that comes with a smaller device and not much space available to put them on.
My future plans:
- I still like the small form factor when using native lenses.
- I might switch to Canon depending on how well the AF works on the M5 or later models.