I'm currently using the 70-300 IS - bought to replace the very good but heavy Sigma 100-300 F4 I had before. At the moment I'm still in the process of evaluating it. It's certainly a big improvement on the old 75-300 IS which was somewhat soft I seem to remember. Last year I had a 70-200 F4 L and I think I'd have to go along with the opinion of others and say that is the better lens but a bit short for wildlife of course. I generally keep the 70-300 IS in mode 2 and it does work very well for panning - but IS is of less use for things like birds than it is for static subjects. AF on the 70-300 [used on a 40D] is OK but not as fast as I was getting the with the HSM of the Sigma.
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