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The Digital Picture is a great resource and Bryan and Sean are very helpful and knowledgeable folks.
I'll throw in my 2¢ here. I got my 60D almost 2 years ago - a friend had one and a 17-55. I was positive I was going to get the 17-55, I used his and loved it. But literally as soon as I pulled the trigger on the 60D body, the price of the 17-55 went up to well over $1k. I had other lenses, but really wanted this one. Then the Sigma 17-50 f2.8 OS went on sale for a very good price. I already had a Sigma 10-20 and loved it - so I wasn't scared of 3rd party brands. I picked it up figuring I'd probably wind up returning it - and half hoping I'd see the 17-55 come down.
As it turned out I loved the lens, and am convinced it's as good or better than the 17-55 as far as optical performance. My friend thought so too, and sold his lens and 60D shortly thereafter. I could have gotten his 17-55 then and been within my purchase window - but kept the Sigma!
Advantages: Small, light, fast and excellent optics. Good color and contrast. A bit soft in the corners wide open - but I think the same as the Canon, and wicked sharp at f4-8. Cons: No full time manual, a short focus ring throw (about 45 degrees) and a hard stop. No focus scale window. I also have found that my AF is dead accurate and fast. I have had no issues with OOF percentages. It just works.
I recently picked up a 6D, 24-105 and 17-40. The 24-105 has left me somewhat unimpressed - perhaps because of high expectations? I can say with certainty, the Sigma is far sharper than the 24-105 on the 60D, and in fact comparing the 24-105 on the 6D. The Sigma is just plain old sharp. I really like the 17-40, far more than I thought. The 24-105 and 17-40 are reversed in how i thought I'd feel about them. Again, the 17-40 performs well on the 60D, but nothing like the Sigma.
So what? I'd say I really recommend a lens made for APS-C. Even though there's that supposed "sweet spot" you get using a FF lens on crop, I'm not seeing the advantage. I highly recommend the Sigma or the Canon 17-55. If I were to have 2 complaints about the Canon (which I still like a lot), it's that it's big, and it sucks dust. I've seen the dust problem - which in theory can be circumvented by using a protective filter. I'm really not big into clear filters unless I have to, but you may be different. The downside of buying APS-C only lenses is of course when it's time to move on.
The other obvious choice is the 15-85. It's a fantastic lens, perhaps down only a notch to the 17-55. If I were to complain about it, it's that it's variable aperture. I have the 17-85 precursor, which is a decent lens but nothing special. The nice thing about the 15-85 is it's great size and fantastic range. A good do-it-all. But the difference between an f3.5/5.6 and an f2.8 is huge.
The Sigma 17-50 was recently on sale for about $430, but I believe the regular asking price is around $550-570.
Oh yes, one more thing. If you look at the resolution charts on The Digital Picture, you'll see that the 17-55 doesn't perform particularly well. Bryan addressed that somewhere where he talks about how he does the tests. He says that for some reason the lens just did poorly there, and yet other lenses perform better. Just to say take the sharpness charts with a grain of salt.