This was the second lens I bought (the first was the EF 28-105 f/3.5-4.5 ii) and it served me well while I was learning. Now I've moved on and up to the EF 200 f/2.8 L and the difference is striking.
I use my long lenses for high school sports (soccer and baseball). The USM drive in this lens is quite capable of keeping up with sports.
The maximum aperature of f/5.6 when extended really hurts though. In all but the brightest sunlight that forces me to shoot ISO1600, which is decent on the 10D, but not spectacular.
The low contrast, however, is a killer for me. Since I picked up the 200 f/2.8 L I haven't taken a single picture with this lens. The colors and sharpness of the prime put it to shame.
If I had it to do over again, I'd pass on the zoom, useful though it was, and save up for good fast primes.