I have a Canon 40D and a 24-105mm f/4.0L IS USM Lens in shipment right now... I figured it was worth the extra $250 for the 40D over the XSI. But to get to your question, I chose this lens on a friend's recommendation and review from
here.
My choice for this lens is twofold: everything I've read points to the lens being more important than the body, and if I'm going to buy a lens I'm going to get the best optics I can (otherwise you'll be in that syndrome of 'I wish I bought that one'). 24-105 is a good all-around, and I'll shoot with it for a while to get the feel for what other lenses I need to accomplish the type of photos I want to shoot. I already feel I'll need a wider aperture one for low-light, but not sure of the range.