So I don't really know why I did it, but I picked up a couple of used Sigma lenses recently, a 70-200 HSM II and 150 macro. Got them both for very good deals and both seem to be pretty good performers.
The macro is excellent. The 70-200 is nice, but not quite up to the Canons, though it's a bang for the buck.
I've thought about the big zooms too. I may keep the 150, but I'll probably end up selling the 70-200, as I have with all the other 70-200s I've bought. Just can't seem to find one that I like enough to stop trying others...
I'll be getting a 70-200 IS II well before I drop $3 grand on the Sigma. I don't really need a longer telephoto... though as my purchase of these lenses demonstrated amply, need doesn't always have anything to do with it.