I just bought a used 1D Mark III and, coming from a 1D II N with, at times slightly wonky AF, this camera is absolutely fantastic. Autofocus is quick and accurate, it will find tiny song birds, hidden in the clutter of a tree without a problem. It focuses in low light (of which there is a lot at the moment) and as far as AF goes it's in a different league than my old 1D2n. Clearly my 1D2n was a bit of an anomaly in that area as it is known as a camera with superb AF. Well, mine had a real problem with small birds in a tree against a light background; the camera would focus all over the place but not on the bird. Obviously this was the first thing I tested with the 1DIII and it has no problem at all with it.
Another thing I'm impressed with is the noise. I'm getting less noise at high ISO speeds (1600 and higher) and the noise is finer and easier to deal with in post.
I love the colours this camera gives me. They seem just a tad richer. I'm not very technical and don't know if it's because the 1D3 is 14 bit depth as opposed to the 1D2n being 12 bit, or the 2 mega pixels more, but I have noticeably more wiggle room in post.
So far I am happy with my new camera. It does take some getting used to though. You would think that after shooting with a 1D2n for 2,5 years, a 1D3 would be no different, but it is. Everything is different: ergonomics, user inter face, weight even. But it is wonderful that everything is there at the touch of just one finger. There is also a dedicated ISO button which is a true blessing and I have my joystick back; I had one on my very first camera, the 40D and missed it dearly on the 1D2n.
I am still just exploring the camera and the weather is not helping, but even with dark, dull light and high ISO speeds this camera can perform. I can't wait for Spring and good light!
EDIT: I need to add something. The first 1D3 was not focusing properly. It did reasonably well with stationary birds but as soon as a bird was moving the camera could not focus properly. Birds in flight, gulls fighting in the water or on land for food, everything was out of focus, although whilst shooting the AF was or seemed spot on. I also thought that with stationary birds the shots were not as sharp as I was used to. Examining my shots, I found that the camera was front focusing or back focusing or side focusing. Pretty bizarre all. I tried out different settings, including those from expert bird photographers, thinking that what worked for them would work for me. It didn't. I tried Micro Adjustment. Also made no difference. I then read up on the 1D3's AF drive, educated myself thoroughly on Custom Function Menu III, but to no avail. In the end I could come to no other conclusion than that my 1D3 had a faulty AF system. I went back to the shop. They believed me without questioning and gave me another 1D3 and this one is brilliant.
What is remarkable about that first malfunctioning 1D3 is that its serial number is way out of the affected range of 1D3's with a hardware AF problem, that Canon provided a fix for. And what's even more remarkable is that when researching my AF problems, I found other reports of people experiencing the same kind of problems, also with serial numbers much newer than the faulty batch. I think this is something to look out for when buying a 1D3. I have shot with four 1D3 now. The first was three years ago. It was a loan and it was perfect. The second and third had a severely malfunctioning AF drive so I had to return them. The fourth is the one I have now and it is perfect. So that's a 50-50 rate. That's not good.