I chose the 70-200/2.8 non-IS because a)IS was prohibitivly expensive b)IS wasn't important enough to me to gather that much more money. Sure it would have been nice, but the choice was between having the lens and not having it, at least for long while, and I don't find that for what I shoot that there is a great need for it. And allegedly it's sharper than the IS, though I doubt that that is much of a big deal. I went with the Canon over the Sigma because I want to squeeze every drop of AF speed out of the thing for sports, and I have pervasive fears about compatability that are probably unfounded (and a bad family history of having to get "THE BEST"). I'm very happy with my purchase.
Remember, also, that at every level you will have people trying to convince you that the more expensive option isn't worth it because they couldn't/wouldn't pay for it. Figure out what suits you best, rather than other people. For me, IS wasn't that big a deal, but I'm not going to sit here and tell you that you wouldn't need it, or that people who use IS are bad photographers, etc etc.
