Yeah I own a mkIV and it is just fantastic for any type of moving subject. I have had a lot of issues with miscalibrations of the body itself, and also lenses (an easy fix). The body HAVE to be 100% spot on calibrated, which mine haven't been. But now that it is, I have no words for just how great the AF works. I use a lot of fast primes ( 24 1,4 II, 50 1,2 , 85 1,2) and they all need to be very accurate or else the images are misfocused.
And the AF is just the base of the camera (although the single most important thing). Everything else is what you need, want and much more. You can set it up just like you want, which makes it very easy to use. Built like a tank, and weather sealing tops it off. Now that mine is properly adjusted, I will NEVER trade it off. I owned the mkIII and people tend to say it's everything the mkIII wasn't, but that doesn't cover just how much better it works than the mkIII.
It will be the best tool for sports, and any other type of photo, conserts, studio-work, it delievers fantastic files, combine this with Lightroom 3,2 and you'll have so much detail and great files that can take any abuse.
I miss the battery capacity of the mkIII though (3500 images pr charge) vs 1200 on the mkIV.
I can't think of one negative with it... You need to adjust and calibrated both body and lenses to that body, but once that's in place, you'll have so much fun with it, and the best part, you can just trust it in any situation.