I've had my 17-55 2.8 IS for about four years now. It was the second lens I bought after my original 450D purchase. The 450D, the (awful) Sigma telephoto zoom and the 18-55 kit lens are all long gone. The 17-55 is, basically, welded to my 50D. It used to be welded to my 7D but that spot has gone to the 70-200 2.8 IS Mark 2. The reason this lens has stayed around and other lenses have either been bought and sold on or bought and stored is that it is so damn flexible.
On a crop sensor (or Short body - hence the -S) this lens has an effective focal length of 27-88mm which isn't so very far adrift from Canon's 24-70mm. What you get with the 17-55 is an f/2.8 lens AND Image Stabilisation. In short the perfect combination.
The only downside I have with this lens is that you can't take this lens with you when you migrate to a full frame body. This is because you can't mount an EF-S len on a FF body. Sure, there is a f2.8 L lens and an L lens with IS at around this focal length for full frame bodies but there isn't a EF lens with the 24/28 to 70-ish length at 2.8 with IS. It's not there.
This lens is one of the reasons I have stayed in APS-C land - the cost of the FF body plus the replacement lens MINUS either the f2.8 OR the Image Stabilisation is too much to bear.