Well I havent used the 18-200 Sigma OS... but I will give you my overall opinion on sigma and my experiences.
The Canon 28-135IS is a very effective lens for a carry around both on 1.3x and 1.6x cameras. I currently have it in my line up and it does a great job, after using it a bit the only downside I have is that the IS is a little jumpy when shooting in Ai-Servo which I do almost all the time.
Sigma is a great company, and has great customer support. With a newly designed lens I would have no quams about buying one, and I am still considering the Sigma fisheye over the Canon because of price. Sigmas only downside is that if you are buying a lens to keep "forever" you may find you will have to send it in periodically to be rechipped (which they normally do for free) because obviously Canon is not going to give out build secrets to Sigma so they can design lenses. So with that everything Sigma does is one step behind Canon since they have to reverse engineer their products. Does that really make a difference? Not really as long as you get the newest updated lens (if you are using a new camera). I think before purchaseing this lens I would see if you can find a store that has them in stock and try them out on your camera. At the minimum I would take a CF card with you and try on a similar camera. Take the results home and take a look to see how the curvature is on the wide angle, and see if its acceptable to you. With that much of a range you *may* find some extra softness on the long end which seems to be the way when you go wide to tele. I personally have never felt it was terrible on any lens I have had, but its all personal preference when it comes to spending hundreds of dollars.
Hope that helps!