I use this lens a lot. It's on my T2i unless I go to another lens for some specific purpose. I'm consistently amazed at how sharp and versatile it is. For the person saying it doesn't get really sharp until f/8, that's not what the charts say -- it's best around f/5.6 and falls off a bit in either direction after that. I often tend to stay around f/8 for DOF and often go to f/11 where it's still sharp enough. I'm surprised by folks that say they have no creep on theirs -- I thought it was just a price to be paid with this lens model. Anyway, a few observations out of experience:
1. The 15-85 often does not allow me to compose in camera. In some situations I have to go too wide to get a wider aperture needed for the shot. Then I have to crop in post. I did not have that experience with the 17-55 as it is f/2.8 throughout its zoom range. I'm consistently torn between these two lenses for that reason.
2. The 15-85 has a 4-stop IS that allows me to go down to 1/10 sec handheld as long as the scene has no motion. When the train comes by here on an overcast day, I have to go to a faster lens. I'm not sure, but I think the IS in the 17-55 is the older 3-stop. Still great, and I've done some really low light handheld with the 17-55.
3. I'm no lens expert, but to me Canon seems to have designed the 15-85 to be an APS-C rough equivalent to the 24-105 on FF. The 17-55 seems their answer to the 24-70 on the FF. At least that's how I see it. I do think if you plan to be serious about photography with an APS-C sensor, one of these two lenses is a must have. And my wholly uneducated guess is that neither lens will change much over the next few years.
4. Macro capability. The 15-85 is not a macro lens, per se. However, in my experience, it has often rivaled my EF-S 60mm macro in image quality. Now, I'm no macro person, but part of the versatility of the 15-85 for me is a very close focusing distance and extremely sharp close-up images. You can be walking around with the 15-85 and when one of those flower/insect kind of things pops up, you haven't sacrificed a lot not having the macro lens along (at least with good light).
Like people, all lenses have good and bad in them. Overall, I say the 15-85 is about as good as a general purpose zoom gets. If that fits your needs and budget, no reason to hesitate.