Mmm... or how about buying the 24-105mm if you don't want the edge softeness or the CA? Perhaps the weather sealing would be nice or the much faster and quieter autofocus? Maybe full time manual focus?
I've actually just bought a Sigma 17-70mm to put on my old 10D and use it as a camera to keep in the car. It isn't a bad lens. I was quite impressed by the build given the price. Certainly a Canon lens at this price point would be much flimsier. Does it have the same finish as a Sigma EX? Superficially it looks similar but it doen't bear comparison to that on my two genuine Sigma EX lenses (120-300mm f/2.8 and 500mm f/4.5). The optics are OK... pretty good really for the money but L busting when examined closely? No. Mind you I didn't expect them to be. All in all I am quite happy with the Sigma;it will do what I want and is ideal for the application I have in mind.
The bad news? It took me three copies to get one that didn't have serious back focus issues. Thankfully I have a very understanding supplier (that might be down to the amount of money I've spent with them over the years ) but even so, having to test and repackage 2 lenses before I got one that worked right was a pain. I also noticed that all three copies of the lens show reluctance to AF if you start off with them really out of focus when pointing at a low contrast subject. Give them a little helping nudge by manualy focusing them somewhere near and they are fine but without full time manual focus override this becomes a little fiddly.
You gets what you pay for and if you can find a good copy, this lens is a bit of a bargain.