@ ziyius: The only thing I've used this lens for (rental) is shooting high school football. Last year I used it on my 30D from the stands and had to crop a lot of my shots. This year I used it on my 7D from the sideline, and I ended up cropping some shots, wishing I was a little further back on others, and being just right on most. With your 1Ds MkII, I would expect you would do just fine on the sidelines of a football game with the 300mm.
Indoor sports would be another matter. I've shot basketball successfully with my 7D and the combination of a 17-55mm and a 70-200mm. I'm pretty sure the 300mm would be way too long for basketball, probably even on a FF body (unless you're up in the stands only, in which case you'll be clonking people on the backs of their heads).
So take that all for what it's worth. Hopefully it's at least a little helpful.
