IS is very nice to have in a zoom lens. For your Rebel XSi, I'd recommend the Canon 55-250mm IS lens as a first zoom. It's not terribly big, and it's inexpensive. If you bought your camera as part of a kit with the 18-55mm lens, you would have a nice range of focal lengths covered. That lens is perfect for outdoor shots of kids, pets, and wildlife. The IS works really well, too.