I just posted this in another thread (which is exactly the same question/topic);
I just made the exact switch myself, but I did it because I really wanted a smaller camera and focus peaking since I have entirely MF lenses.
So far so good. The EVF takes some getting used to, but it's not as bad as DSLR purists would have you believe. I obviously can't speak for AF performance, but the camera does work well for MF (though honestly I don't think it's any better than the 6D with an EG-S screen was despite what people tell you).
I already find the controls faster and more intuitive than my 6D thanks to all the customization, that's a pro I didn't expect, but the camera is a pleasure to shoot with. So far the files haven't blown me away (been wearing my flame suit a good while now
) but they are nice, below ISO3200 I'd say they look similar to the 6D, though the colors are very different and take some getting used to. The extra resolution is pretty cool and you do still get lots of detail at high ISO like with the 6D, but colors don't hold up nearly as well in my experience so far.
If you can make the trade like I did (after selling the 6D I only had to put in about $150 to get the a7R) and you're super curious than I'd say it's worth trying out. You can always resell the a7R and go back to your 6D if it doesn't work out.
I'll add; I don't the there's going to be an a7Rii personally, I think the a9 is going to fill that gap. Hopefully I'm wrong, because I really like the way they've lined up their cameras now and I still have my eyes/heart set on an a7S.
And for what it's worth, I paid $1350 for my a7R and it looks almost new, one tiny little scratch on the body right by the Sony logo, but it functions perfectly. I saw a couple more listed used for around $1400, so you should be able to get one around that price if you're patient.