As most have said, this is a must have in many bags. Not only for the sharpness of the photos it can produce but for lack of space it takes up in your bag and the price!
I have the 20D, so I can't say that I use this lens as a walk-around, the 1.6x crop is just too much for indoor group photography, which is what I tend to do a lot of. However, for working outdoors at party's and such this lens comes in very handy. Also, this is a great lens for portrait shots of 1-3 people, or even for pets. I also used this lens at an indoor soccer shoot which worked nicely because I had enough room for composing photos and I had the F/1.8 to work with the light! Photos came out very nice and the AF actually kept up!
For the few times that I have used this lens on the 20D, it has given me such quality photos that I have no regrets about the purchase.