Liz,
I've used the 17-55mm F2.8 IS; the 24-105mm F2.8 and the 28-135mm F3.5-5.6 IS and they're all pretty good lenses. Of the three, the 17-55 is the sharpest. I currently use the 28-135 IS for a couple of reasons:

1. it is sharp stopped down to F8.0.

2. it is a little soft wide open.

That means that for most show portraits -- which are taken at F8 and 1/200, I get sharp results. For most studio shots of people, I can open it up and get some nice "soft-focus" effects which are very flattering for skin. And again, in the studio, I can stop it down for dogs and get very sharp results.

If you can control your lighting, the relatively slow maximum aperture isn't a problem and outdoors the IS gives you a good 3 stops of hand-hold-ability. For the money, I'd go for the 28-135 IS. It's a much better lens overall than the 17-85 kit or 18-55 kit lenses.

Jim