If sports shots and portraits are what you want to shoot then the 70-200 f4 L is probably the way to go. I have used this lens and as previous posters have said, it is a phenomenal lens. Probably the best value L lens there is.
Personally, I shoot landscapes so this lens isn't the best for me. As I have a 1.6x body (a 40D) I think the best landscape/walk-around lens has got to be the 17-40 f4 L, just for value alone. It's about half the price of a 24-70 f2.8 L, much smaller and lighter, and on a 1.6x body, the range is roughly 27 - 64 which is comparable to the 24-70 on a full frame body.