Good choice - the EF-S 17-55mm f/2.8 is an excellent lens, and is the one that's on my 7D most of the time. It will definitely focus better in low light than the kit lens. Indoors in good room lighting it's a good performer, but in relatively dim light, though IMO it focuses fine, f/2.8 is often not enough without bumping up the ISO pretty high (for me, a shutter speed issue with moving kids as subjects - for still subjects it's fine with the help of the IS).


So, the 17-55mm is the place to start, but if you end up taking a lot of ambient light indoor shots you may find yourself adding a prime to your kit down the line. The EF 85mm f/1.8 does a bit better in low light, and I love it for head/shoulders shots indoors in ambient light - but, I sometimes find myself needing a shorter focal length so either the EF 50mm f/1.4 (waiting to see of if there's really a MkII) or the 35mm f/1.4L will be in my future. But even then, I think the 17-55mm lens will remain my most-used lens.