Most of what I would say about the a7R has already been said elsewhere, so rather than regurgitating the same stuff here, I'll give the reasons I bought it and the reasons I sold it.
I bought the a7R when I realized it would be almost a direct trade to it from my 6D. I had an X-E1 alongside my 6D and realized I much preferred the form factor of the mirrorless body, so I posted the 6D for sale and bought an a7R. I chose the a7R over the a7 after doing some research on the files and to my eyes the a7R files looked better overall and I didn't want to spend almost a thousand dollars extra on the a7S without first testing out the body to see if I liked it. I bought it because I don't like renting gear because I don't believe a week is enough time to get a feel for a camera, instead I buy used and use it for a month or two (or however long it takes) and sell it if I don't like it.
If you want the short and sweet version it goes like this; I loved the body, I liked the base ISO (100) performance, but I hated the high ISO (3200+) performance and shutter sound more than the things I liked which lead me to sell it.
I've seen a lot of mixed opinions on the a7 body ergonomics and this was my main concern when buying one. I have to say that the a7 line has for my needs the best ergonomics of any camera I've ever used, and I'll tell you why; customization.
Just about every button on the back of the camera is customizable, this goes beyond even the labeled custom buttons (c1, c2, c3). On the back of the camera you can change the function of all 4 buttons on the control wheel, both custom buttons (obviously) and one button with a switch that can serve 2 separate functions. Holy cow! You can set it up exactly how you like it and know exactly where everything is, it makes shooting much faster and more enjoyable for me, even though there are some functions that they don't let you assign that I wish they did.
On top of all the re-assignable buttons you also get a function menu (similar to Canon's 'Q' menu) which is also fully customizable. It has 12 different menu options which you can set up however you want. Being that I'm an MF guy I have all my settings set up for manual focus centric functions (i.e.; peaking color, peaking level, focus magnifier, etc).
Beyond all this amazing customization you can then save multiple sets of custom functions in the camera. For example, if I buy an AF lens or two in the future (which I intend to) then I can set the camera up completely differently to take advantage of different focus settings, then switch back to the old setup with a quick menu switch... winning.
The only gripes I have with the ergonomics are the shutter button, auto ISO usability and a couple other little things. The shutter button issue isn't the location as most seem to hate, I actually like the position of the shutter button as it allows me to easily and naturally (I did this without thinking about it the first time I held the camera) switch to using my thumb on the shutter button when I'm shooting below eye level using the flip LCD (another thing I love). The thing I don't like about the shutter button is that it's mushy and vague, it's very hard to tell exactly when the shutter is going to fire and this can lead to firing off shots before you're ready or causing the focus magnification to turn off prematurely since it turns off as soon as you depress the shutter the slightest bit.
The auto-ISO issue is that the camera will not allow you to set a minimum shutter-speed when using auto ISO... this is a massive and imo ridiculous oversight. It defaults your minimum shutter-speed to 1/60s when using A mode with auto-ISO, not nearly fast enough most of the time. Thankfully you can walk around this issue by just shooting in M mode with auto-ISO enabled and using the exposure compensation dial to adjust the ISO level as desired (something I always wished I could do with my 6D).
The other minor gripe I have are with the focus magnifier function. The issue is that it requires 2 button pushes in order to activate. This might not sound like a big deal, but it's very frustrating when it's something you like to use a lot, especially since with other cameras (like my old X-E1) it only takes one click of a button, or in the case of my Magic Lantern enabled EOS-M all it takes is half pressing the shutter button.
You've already heard, it's fantastic. The images are great at ISO100-800, from color to sharpness to DR, it's all there and it's all good... sometimes too good. I learned quickly to downsize all of my shots to 16mp images because the body was clearly out resolving some of my lenses, lenses which looked very sharp on my 6D were less impressive on the a7R, I think this is partially due to the slightest miss in focus being much more obvious on the a7R due to the extra resolution.
You've also likely heard of the "mirror-slap" issue with this camera, personally I don't believe that's really the problem. I think it's more likely that the extra resolution is showing any flaws you create, such as camera shake in general. The body is extremely light and the shutter is extremely vague, this leads to a slight jerk often when pushing the shutter button. I found that I had to shoot at least 1/100s to get sharp shots using my 50mm, with my 6D I could often shoot lower than 1/60s without a problem.
Here's some samples from the a7R at low ISO
IMAGE LINK: https://flic.kr/p/pVhYX6 DSC01864.jpg by EverydayGetaway, on Flickr
This is the main reason I sold the camera. I like to take and use my camera pretty much everywhere. The main reason my X-E1 wasn't "doing it" for me was that I wasn't comfortable with it's shots beyond ISO3200, and was hoping for more from the a7R. It honestly gave me less in some respects. While the noise amount and pattern aren't too bad on the a7R (still not as good as my 6D was) it's the way it degrades color that ruined it for me.
When shooting color images I'd find lots of color shifting, color noise and just generally muted or blotched colors when going above ISO3200. I had to spend far too much time editing high-ISO shots for the camera to be worth it for me.
Here's the best samples I have at high ISO.
With this shot I had to use the brush tool in LR5 to change the white balance in select areas of the image in order to get it looking at all right and apply a small amount of NR (as I said, the grain doesn't bother me). This shot is at ISO12800
IMAGE LINK: https://flic.kr/p/pJUp3B DSC01554.jpg by EverydayGetaway, on Flickr
I think in B&W the images look great at high ISO. This one's ISO8000
IMAGE LINK: https://flic.kr/p/qfLwFy DSC01202.jpg by EverydayGetaway, on Flickr
GENERAL USABILITY/OTHER THOUGHTS
The other thing you've probably heard a bunch about is the shutter. Well, it is definitely loud... not to the level I was lead to expect, but it's definitely the loudest camera I've owned. The biggest problem I had with that noise though was that my subject would get confused by it. For example, I'd set up the shot and click the shutter and it makes that first distinguished click, then the subject starts to relax and move and then there's that second loud click. I constantly got people asking me if I got the shot, or even if I had taken 2 shots. I would relate it to shooting a DSLR in mirror lock up mode and just clicking the shutter button twice for one shot, it's that same sort of clunky delayed sound.
The menus I found to be perfectly fine. I've seen a lot of negative reviews in regards to all the Sony menus, and imo the NEX cameras deserve all that negative press, the a7 line though, not at all. I think the menus are every bit as intuitive as the Canon menus are. My only issue with Sony's settings/menus is that it pop's up a little tutorial explanation whenever you rotate the mode dial, just like my old T2i Rebel used to do... on a $500 rebel it makes sense, on a $2000 specialist camera it's very out of place.
Overall it's a great camera for a specific group of photographers. If you want maximum IQ under ideal conditions like in a studio or on a tripod, you'll adore this camera. If you're like me and want to use the camera for many things, you'd be better off with the regular a7 or the a7S, which is what I ended up buying after this camera.