Backing up, there are 3 lenses mentioned above that cover the standard zoom range of wide angle to short telephoto on APS-C - the 17-55mm, 15-85mm, and 17-85mm. Be careful about the last one - it's optically inferior to the other two.

For most, the choice of a top IQ APS-C general purpose zoom boils down to 17-55mm f/2.8 vs. 15-85mm f/3.5-5.6. They are optically similar, except the 15-85 has a bit worse vignetting and barreling at the wide end. Build is similar, too, both use ring USM and have FTM. So, basically it comes down to a constant f/2.8 vs. broader focal range and lower cost.

If you'll use mostly outdoors, don't need to stop action in low light, and don't plan on shooting a lot of portraits, the 15-85mm is a great choice. Personally, I opted for the 17-55mm, and found it to be excellent.

$850-900 used is a decent price, although I've seen them on Craigslist for $800 and a bit lower.

I've said previously that I planned to keep the 17-55mm as long as I had an APS-C body, and especially for times when the AF of the 5DII wouldn't be adequate. But...my 1D X will be here soon, so AF woes with FF are done, and in fact, looking over my EXIF, I'd used the 17-55 just twice over the past year - not enough to justify keeping it. Accordingly, I recently sold mine for $850, not bad considering I paid $1000 for it new, in 2009. The proceeds from that will likely go towards a 24-70 II.