I've been using 5d2s for a long time because I am simply not rich enough (and I don't want the huge body either) to buy 1d-series cameras for work. I've shot about 20 weddings with the 5d3 since I bought it, and I finally sent it in to Canon the other day for an autofocus checkup.
My #1 issue is reception autofocus. My 5d2s beat my 5d3 at reception focusing (with 580EX AF assist always used). They are more reliable for AI servo and faster for one-shot.
My 5d3 struggles to acquire in one-shot, taking probably close to 2 real seconds (it feels like forever) to establish and confirm a lock on a static subject during wedding receptions with lenses like 24L II, 35L, 50L. In contrast, in the exact same lighting conditions and with the same lenses and flash AF assist, my 5d2s all snap right to focus and I click the shutter within about 1 second or less, getting an in-focus image about 90-95% of the time.
With AI servo, the 5d3 is just inferior to my 5d2s in low light. I was hoping, one day, to get a camera that would give me the ability to track subjects well during processionals/grand entrances/reception dances using the outer points, because I'm pretty tired of being stuck with center-point framing for these situations. I thought the 5d3 was supposed to solve that problem, but in fact it took a step backward in low light AI servo accuracy and locking. My 5d2s (center point) both acquire faster and track better in low light than my 5d3 (center point also).
After sending the 5d3 in to get it checked (and so they could charge me $175+), I got a nice copy of the page saying that the camera focus in low light is blah blah etc. etc. use AF assist (which I had noted I did) and the overall feeling was that they were very condescending and disregarding of my equipment issue. To me, a $3500 camera should perform better than a camera that is now worth ~$1300...and so the 5d3 performs arguably better in GOOD light, simply because it has more focus points to choose from which are generally pretty accurate - in GOOD light. However, it performs worse in reception light. Wedding photographers will understand what I mean by reception light.
My 5d2 AI servo is not useless during receptions, but my 5d3 AI servo struggles more in the same reception lighting. My 5d2 one-shot locks quickly and pretty accurately during receptions, while my 5d3 takes significantly longer in one-shot to lock, and sometimes it gives me pretty wild misses, during receptions. Always using AF assist with 580EX, and btw the 5d3 firmware is the latest version (1.2.3 at this time). The camera simply focuses slower during receptions than my 5d2s, and AI servo misses more.
I also don't like that AF points don't light up before focusing, but this is less important to me than a fast, responsive AF system for wedding receptions. I have had to put away the 5d3 on multiple occasions and bring out a second 5d2 for wedding receptions just so I can get good dancing photos, good speech/toasting photos, etc., before all the key moments went by and ohhhh too late, my focus system didn't lock in time...
I have been having this issue with 5d3 performance since I bought it close to a year ago. I do wedding photography primarily, and I have quite a bit of experience shooting weddings and dealing with the various demands of weddings.
Most of my AF experience is with single point, with only a few experiences using the expansion settings. I have never had trouble locking or tracking with single point with my 5d2s as long as there was enough light to do it, and being used to that I expected the 5d3 to be able to perform at LEAST comparably. It does not.
At this point, this critical functional limitation of the 5d3 has left me very dissatisfied with the camera. I thought upgrades were supposed to do everything just as well or better than the previous version, but this newest camera has shown me that more AF points definitely does NOT mean a better (or even comparable) AF system in every situation. Clearly something was traded off when they added a ton of cross-type points, like low light acquisition speed and being able to see your AF points illuminate when you start AF.
I've been playing around with Nikon equipment for a while and am beginning to think that is my forced alternative. I don't like the things I hear about Nikon customer support, and I don't really like the direction they are going with some of their releases or how they handled function issues (focus issues with off-center focus points, oil on sensors, etc.) but Canon isn't innocent of that stuff either. And if I use an all-Nikon setup then I will have excellent dynamic range at low ISOs as well. Too bad Nikon doesn't have the glass I like best (35L, 85L, etc.)...but compromises must be made in some direction, and I'm intensely frustrated with Canon's latest cameras' focusing systems in low light.
It's very specific, mind you: Low light performance is inferior to my 5d2s. In good light, my 5d3 has better AF flexibility for tracking with outer points, and the one-shot focus acquisition is comparable to my 5d2s.
