I haven't used the Canon 17-85. But I now own the Sigma 17-70 after reading those photozone reviews. My first impressions with some test shots have yielded extremely sharp photos, nice autofocus, and I can take sharp macro shots with the extended lens right up at the subject. The build quality is also good, and a steal at 450 Canadian. My only complaint is that the lens hood is very compact, and leaves very little room to properly put the lens cap back on. With a UV filter (or any other filter), it should be a little easier since the fingers don't have to reach down quite as far. That said, the lens hood snaps on and off with ease. For every third party lens I've bought, it came with its own lens hood. I can't say the same for Canon.
I don't believe that Sigma lenses are generally less reliable or compromised than Canon lenses. If these forums are an indication, there seem to be a fair amount of people with the opinion that Canon has quality control issues with *specific* lenses. But this isn't the case for all lenses, and not even all lenses of a certain model. Sometimes people get lemons. My Sigma lens came with a 5 year warranty, which isn't too shabby. Also, I think that for 450 dollars, Canon can't match what's being offered by third parties in regards to APS-C optimized lenses in the non-budget, sub 1000 dollar category. Tamron has the excellent 17-35 Di LD, and the 90mm Di 1:1 Macro. I own them both, and can vouch for quality optics and build quality...
The Canon 17-85 EF-S looks better than their other EF-S lenses, but the build quality doesn't seem to be much of an improvement from their other EF-S lenses (i own the 18-55 kit lens). Speaking purely from my limited experience with APS-C sized lenses, Sigma and Tamron are ahead of Canon with respect to "bang for your buck".
