I've shot mostly Canon primes -- 24 f/2.8, 35 f/2, 85 f/1.8 -- and the primes beat this lens each time in terms of clarity/crispness of image (at the same f/stops). I'm a part-time wedding/event shooter and I find this lens barely acceptable. I've never used a Canon 17-40 or 16-35 so I can't comment on the comparison.
I don't know if I have a below average sample, but I partly based my decision on FStopJojo's reviews on his Pbase site. My Canon 24mm vs Sigma 18-50 comparison isn't a completely fair one as the 24mm exhibits obvious distortion. The 35mm prime -- which I always thought was a bit soft -- seemed to pull more fine detail than the Sigma at f/2.8 and f/5.6 and f/8. Comparing it with the 85mm isn't fair, but it proves again that the 85mm is an exceptional lens.
I'm using it with the Canon XT and it definitely beats the kit lens...but you already knew that. It'd probably make an awesome walk-around lens, but for the same price you can pick up 28 f/2.8 and 50 f/1.8 primes and have much better clarity.
BUT there is one area in which this camera seems better than my primes -- low light focus. I don't know why, but the Sigma seems to be quicker in low-light situations than all of my primes. For this reason alone, I think it's worth using as an event/wedding lens.