I don't know exactly which Sigma lenses they are including but I'm guessing that they are of similar quality to the Canon kit lenses.
Sigma has cheaper consumer level lenses, just as Canon & Nikon do. They also have some really nice lenses but as with the big companies, they don't come cheap. Basically you get what you pay for. Although, sometimes you can get a little more for your money with Sigma because your not paying for the big brand name.
The best thing to do is to search for reviews of the specific lenses. You will often find conflicting reviews but the more you read, the better idea you will have.
I have the 18-55 EF-S, Canon kit lens with my 20D and I think it's actually a pretty good lens. It's very light and plastic but for $150 CDN it's a nice wide lens for my camera. Also, a good inexpensive lens to have it the 50mm F1.8