Thought I'd post these pics up... a pretty crude (real-world) comparison of a Sigma 17-50 f2.8 and a Nikon 17-55 f2.8. These shots were taken at ISO 200, f2.8, 1/8000 second.
These are 100% crops... (D300).
The last shot is a photoshop edit to try to add some contrast in the Sigma shot. It's still not quite as good as the Nikon, but I think it stacks up reasonably well - the Sigma is certainly not a bad lens for the money.
What do people think?