I feel like I need to stand up and defend the 28-135. Its not as bad as most people say. It may be a little long for some people, but I guess it depends on how you like to shoot. I used it almost exclusively at
Disneyland for 4 days and can't remember a time I wished it was wider. The IS is not the latest generation, but I have gotten some great low light shots with it. It may be part of the new "kit" but that is just because it is affordable. And you can probably get one cheap off the web from people who upgraded their body and got the this lens as part of their kit but don't really need it (like Master Mason above).
When all I had was my 18-55 kit and nifty fifty, I thought this lens was heaven. But in the interest of full discloser, after I got my 85 f/1.8 and first "L" and experienced some "real glass" I fell out of love with it a little.
But for someone in your situation who wants more reach than the standard kit, and could use the IS, but doesn't want the cost of an "L" (and hasn't experienced one yet), this is a great lens for the price.
I haven't ever used the Sigma, but in general, the more extreme they try to make the zoom (11.1x for the sigma vs. 4.8x for the canon), the less quality you can get out of the lens.