I had the Mark IV on order with Amazon since Canon's product announcement on October 20th of 2009. Since my local camera shop received the unit on Monday January 4, 2010 and Amazon still listed it as "Not Yet Received" I purchased mine from the local retailer despite the additional $464 in sales tax.

I have been a Mark III shooter since June of 2007. For my professional work, my primary subject matter is live theatrical performance which includes acting as well as dancing ranging from ballet to traditional Tango to the very fast paced ballroom, specifically shooting Burn the Floor, which is just completing their Broadway run this month. Live performance shooting has many of the same elements of sports photography, except the lighting is much lower and at times more severe due to stage lighting. My Mark III performed well in most instances although I did have occasions where the autofocus did not perform as well as I had hoped. But to be fair, I cannot say with 100%... Read more

(Edited to fix my poor grammar and reword a few things to be more clear)

The 1D mk III was my first 1-series body. Before that I had, in reverse order, a 5D, a 20D, and 300D. Each step along the way was nicer and nicer. I couldn't imagine a better camera than my 1D mk III, but now I have it.

Over time (mostly through reading about the camera) I learned that my mk III had poor autofocus. I had an early version with the defect, but also because the mk III apparently did not live up to the autofocus of the 1D mk IIn. I had the defect fixed, and my auto focus was better, but still not as good, or so I had read, as the mk IIn autofocus. I can attest that I certainly felt frustrated with the mk III autofocus on a regular basis.

I haven't been to a sporting event yet, so I can't speak to that kind of focusing, but in good light with a stationary subject my gut feeling is that, yes, focusing is better in the mk IV than it was in the mk III. I can also... Read more

I've had the pleasure of using a pre-production 1D Mark IV for several weeks now, and I barely use my 5D Mark II anymore. As a photojournalist, this camera does everything I need it to -- most important for me:

- Low noise high ISO. I have no fear whatsoever going to ISO 12,800. Images are still sharp and have good color. No ugly lines at all. Perfect for shooting sports in stadium with crappy light (see link below).
- Better autofocus, especially in low light. With any camera I've used before, autofocus on low light was terrible. It's still not great, but it's a lot better.
- 10 FPS, with a large buffer. With a fast card (60 MB/s or higher), you can lay on the shutter for several seconds on highest quality before it starts to lag.
- Compared to the 5D Mark II, this thing is built like a beast. I've banged it around a lot and it still looks brand new.

Here are a couple galleries shot entirely with the Mark IV. I believe the ISO for the first one is... Read more