I have a 600d/T3i and completely agree w/ Neuronatomist. I bought the 15-85 first and find it just the little bit too slow for indoor work. Fantastic outdoor, but really miss the 2.8 light gathering ability.

I went through the 18-55 & 55-250 combo, found that I wanted to change lenses too often, that is why I went to the 15-85 to get that extra length. Now I am wishing I had gone to the 17-55 for the 2.8. At f4 both are really sharp (look at the ISO crops in the reveiw section) in the wide side. The variable f stop closes down pretty darn fast, so you lose the extra stop or two pretty quickly.

I have been fiddling w/ the video side of the 600 and hee hee hee it is alot of fun. I wish I had the 2.8 for the indoor video where flash is n/a and light is just silly looking. I am looking at the 16-35 L for "indoor" work, and keep the 15-85 for outdoor.

Happy clicks

Mike