I have had the original Sigma 17-70mm for over five year and am still very happy with it.
It is a good range, roughly equivalent field of view to 24-105mm on a 35mm frame. A minimum focussing distance of a few inches, an underrated feature in a general purpose lens. And a maximum aperture of f/2.8-f/4.5, which is quite useful on a lens with variable range as it gives more flexibility in low light, but when stopped down is still reasonably wide.
The only downside is the focussing is a bit slow, but the current model upgrades this by adding an HSM motor, as well as stabalization, and a wider f/4 aperture at 70mm.
I have made 20x30" prints with this lens and an 8mp Canon 30D and they look great. I would definitely recommend it as an option. It is almost half the price of the slower Canon 15-85mm, and for me that aperture makes it difficult to justify despite the greater range.