I can't say anything about the IS version, as I've just purchased the 70-200mm f2.8L USM. As for sports I find the IS turned off produces a much better shot. Don't know why, but a journalist at the track gave me that tip last year and on the f4 it made a surprising difference to the shots, with the IS turned off, they were sharp.
Compared to the f4 I've ust sold, there is quite a noticable difference and jump up in quality. The pictures it is producing are bright with extremely good colour (I've not had to adjust anything in PS) and the sharpness of the subject and the lovely background blur are something else. Even hubby is impressed and looking at my shots said it was money well spent
If you want to see the shots I've done with it, look at my site and under time trials 2008 check out the Welland Valley event.
The chap who bought my f4 is selling an f2.8 on ebay this week, not sure why! Unless it's a weight issue for him, but to be honest on my 40D it's not that heavy & I don't think it draws that much attention, but then when I'm out at major cycle races the pros have them anyway & it probably helps the cyclists see where I am so they can smile.