I'm in the market for a telephoto zoom and could use some opinions... I have a Canon 50D and a Sigma 18-50 f/2.8 (and a Canon 50mm f/1.8, although that's less relevant), which was an upgrade over the Rebel XSi w/ Sigma 18-200. The 70-200 2.8 is more than I need in every way (too heavy, too expensive, faster than I *need* even if not faster than I would *like*). The 70-200 f/4 is a great weight, easy to walk around, not to mention all the other perks of L glass. But if I'm going to go for it, I think I should go for the IS version, which will help when I am indoors (mostly I expect to use the new lens outside). The 70-300 USM f/4-5.6 IS also appears to be a very solid, good quality lens. Obviously I lose some environmental sealing, the fixed f/4 and probably some sharpness (although I've gathered this one is still very sharp), but I gain zoom to 300mm and an IS lens for about the cost (slightly less) than the 70-200 f/4 L non-IS. I guess I'd be considered an "advanced amature," I don't make my living on photography, but I am trying to buy equipment that will last a long time and serve me very well (I have a 6 month old who is a common photo subject now!) for the foreseeable future. I know a lot of the decision here comes down to personal preference, but I want that preference to based on as much knowledge as possible! Opinions? Am I missing a lens a I should consider? Anyone else face a similar decision?