According to his analysis, the non-IS version is only noticibly sharper if one is shooting film. In digital, he says it makes no difference, however, I suspect that if you were shooting at a slower than optimal shutter speed, without a tripod, and using the non-IS lens, I think you would see a difference. You would have to decide how important that is. For me, I shoot at the beach often at sunset, under low light, and while I can expect to be able to shoot at 1/125 or even 1/250, having the IS available seems to give me good results on the limited amount of shooting I've done so far. When I was shooting Nikon/film, I would switch to a lighter, faster fixed-length (an 85mm f2, or the 105mm f2.5) to limit camera (and my hand) shake under low light, but I didn't need to do that with the Canon 70-200 with IS. (And I don't know if it will make any difference, but you can turn the IS off).
Regards,
Andy Pearlman
Andy Pearlman Studio