Good indoors and out. Sharp, saturated images from edge to edge, on APS-C as well as Full Frame bodies. Very fast, deadly accurate autofocus, even in low light. The bokeh produced in shallow depth of field shots is amazing.
I earn my living with my cameras, and I can depend on this lens to deliver consistently sharp images under any conditions. My wife is frequently my second shooter, so I bought one for her as well. She rarely takes this exceptional glass off of her 40D.
Some say the weight is a disadvantage, but to the contrary, I find that the mass of this lens helps to hold my camera stable. I took hand-held shots in the very low light of Ripley's Aquarium, and most were keepers.
The zoom extension moves inside of the hood when extended. The lens is cleverly designed such that it is fully extended within the hood when at its widest angles, and retracted at the tele end. This keeps the front element positioned toward the front in the hood perfectly, preventing vignetting at wide angles. When at around 70mm, the front element is deep within the hood, as it should be.
The only thing I will knock this lens for is the price. But then, I think everything is overpriced. I can't argue with the fact that I indeed got everything that I paid for when I purchased two of these lenses.