I've been looking at the Sigma 100-300mm EX F4, gets good reviews on all the usual sites like
www.photozone.de. Most people reckon its as sharp if not a bit sharper than an equivalent prime, even one from canon.
I think i can live with the f4 instead of f2.8 as most of the time i'm gonna be using it outdoors.. Was looking at the Canon 70-200mm but the SH prices are a bit steep for my liking (or currently stingy pockets
)
Good bit about the sigma is that with a 1.4x tele conveter you get a 420mm f5.6 lens, so still get AF and its still pretty sharp.
I use a canon 5D but i do a lot of commercial/fashion work that suits having a full frame camera better.