For me, the only thing keeping me from buying this lens is the price, not the lack of IS. I'm not saying that it's overpriced. I just don't have that kind of money right now to justify upgrading from my 24-105. and to be honest, if I did have the money, I might spend it on a vacation rather than a glass upgrade at this point.
Again, for me and the way I shoot, IS on a lens that only hits 70mm at its max isn't a necessity. It would be nice to have, but I would have to factor in the added weight and cost if there was a Canon 2.8 IS lens in the mix to choose from. I understand that there are a lot of people who would disagree, and that's fine.