OK, I've been looking at 70-200mm lenses for a few months, hired the Canon f/4 IS and the Canon f/2.8 non-IS and still haven't made up my mind!
Got to the decision that it'll defo be f/2.8 but possibly not IS, so that leaves the Canon, Sigma and Tamron. I like the pricing of the Sigma/Tamron (and reviews suggest the quality's pretty good) and the close focus of the Tamron (the Sigma's not so good). The Canon's got a better build but is that stupid colour - and the close focus is OK on the new IS (but don't like the price - and so I'm not convinced that it's very cost efficient!)
Useage: 20/30/40/7D for occasional sport, more often canines and closeups of butterflies and wildlife