Hey,
I have the 15-85mm Canon lens and it is good. The range is fantastic, IS works really well. However, I do find it slow indoors. I have since paired it up with at 580EX-II and have no issues. However if I went back, I would make sure my mid-range lens has a constant f2.8 aperture, even if it meant going to Sigma/Tamron and/or splurging a pit more money.