I picked up a 5D3 last Friday and had my first job with it on Saturday (I had to figure it out quick on Friday night). The job was three dance recitals, 1.5 hrs each. I shot about 3,000 photos throughout the day.
I also brought my 5DC with but only took about 100 photos with it and threw most of them out for either bad focus or too much noise.
I started shooting at ISO 8000 but eventually settled in at about 5000, shutter ranged from 1/400 to 1/640, mainly 1/500, and I shot at f/2.8 with a 70/200 IS and a 135. I did use a 24-70 a few times for large groups.
The focus is amazing. I've used a 5DC for years, and a 40D for sports/action, and this is just the best I've seen. I only threw out a handful of photos for not focusing, and most of them I knew right away I missed. I use back-button focus and either the subject moved away from the focal point or I just hit a spot in the background. Almost never was it the camera's fault.
High ISOs are stellar. I only shot 6400 on the 5DC when I absolutely had to. The ISO 8000 (and a few at 25,600 when the lights were low) were all very usable. Even in the extremely low light the focus was spot on.
I need a grip. I'm used to shooting with a grip and without one this thing was killing my hand. I also had only two batteries with me and they only lasted a little over one show. I had to charge one during the second show.
Lightroom 4 RC2 slows to a crawl with these photos. I think it's an RC2 thing, but it is painfully slow, like 8-hours to export web sized images for 900 photos.
The files are huge. They are twice the size as my 5DC files. I had to dump my memory cards to a laptop between shows. I actually had a bunch of 4GB cards with me but didn't want to use them since they only hold about 100 photos and I'd be switching too frequently. I really hate to format memory cards and reuse them at a job. I'm buying more for future events like this.
Bright! Wow, this viewfinder is bright. When I picked up the 5DC I could barely see the stage. No wonder I've had so many frustrations with that camera. Looking through the 5D3 viewfinder was like watching a HDTV. It was really amazing. In the 5DC's defense, it does have an Ee-S focusing screen so that may have made it a bit darker.
Overall, I really love this camera. My photos are crisp, clear, and have very little noise. The colors are also bright and accurate.
I'm very happy that I was able to get this before this job. My results would be a different story if I had used only the 5DC.
I know a lot of this has been said over and over, but there you go.