So, finally got around to picking up a Canon 5D III. Way past due, been meaning to do it forever, just...gotta scrounge up the funds, you know? First it was the 600mm lens, then it was a crapload of astrophotography equipment (kind of unexpected, but I realized I'd been putting off astrophotography for YEARS, way longer than the 5D III, so I decided to just dive in head first).
I'm pretty happy to have the 5D III now, though. Now with almost five hours strait using the 5D III on the kinds of things I generally shoot (I'd only used it by borrowing another guys out in the field, or in stores before), I have to say...compared to the 7D, the 5D III is EFFORTLESS. It just works. No fuss, no hassles, no fiddling with the AF system or anything like that.
For all that people croon (and scream) about the IQ benefits of full-frame sensors, the SINGLE biggest and most immediate benefit I saw with the 5D III was focusing. It is BLAZING fast, locks in instantly, seems to intuitively just know what thing you want focused, even when there are potential obstructions (right out of the box, it was focusing on deer through foreground tree branches and other obstructions without any effort on my part), and it nails it over 90% of the time. I've noticed a little bit of jitter a couple times, but no where even remotely as bad as what I experienced with the 7D. There is the full-frame IQ benefit for sure as well. I haven't noticed it quite so much in my subjects themselves, however background boke is AMAZING. It used to be so noisy with the 7D...and difficult to clean up without greater measures than just the Lightroom NR tool. The 5D III has so little noise in the background, and it cleans up super well.
I do feel the frame rate difference. It sounds a lot slower, and feels a bit slower. I am quite certain I'll miss the 8fps of the 7D. I can also tell that unless I fill the frame more, there is a slight loss of resolution. A number of the birds I shot just ended up so small in the frame (I forgot to take my TCs with me as I never really used them with the 7D, and at the moment I have no idea where they are), and while they don't look bad when cropped, they definitely don't have quite the same detail. I think that will be OK, though...once I find my TCs, I think 840mm f/5.6 will become the sweet spot, and if I need it, 1200mm f/8 will completely close the magnification/resolution gap.
Most of my shots so far are ever so slightly out of focus due to not having run the camera through FoCal yet (I need to move my license over to the 5D III), but I don't think you'll be able to tell at these sizes. Anyway, here are some of my first bird shots.
Snowy Egret
Smallish wading bird, brilliant white with a black bill, yellow facial patch and yellow eyes. Tall, with long black legs and yellow feet.
The sequence of photos here is out of about 1500 I took (the first 1500 with the 5D III). I chose this particular sequence to share first, as it shows the ISO capabilities pretty well, ranging from ISO 400 to as high as ISO 6400. ISO 6400 on the 5D III is easily as good as ISO 1600 on the 7D, and the way the noise cleans up, it's maybe even as good as ISO 800 or somewhere between 800 and 1600.
This particular Egret was a skilled and prolific fisher. It must have caught a dozen or so fish wile I was photographing it. Some of them were so large I was surprised it managed to squirrel them down it's gullet!
Canon EOS 5D III
Canon EF 600mm f/4 L II
Gitzo GT3532LS + Jobu Pro II
1/2000s @ f/4 ISO 400
1/160s @ f/7.1 ISO 640
1/500s @ f/9 ISO 800
1/125s @ f/8 ISO 1600
1/400s @ f/8 ISO 6400
^^--- OH HELL YEAH, BABY!! --^^
ISO 6400 kicks ass on this camera. SO much more color fidelity and way less noise.

No way in hell could I have ever gotten that shot, that late after sunset, with the 7D (it was probably 15-20 minutes AFTER sunset, so quite dark in the grand scheme of things).