Tamron's 17-50 VC isn't anywhere as sharp as Canon's 17-55. In general, Canon is usually the way to go. My strategy is to pick the best I can buy and if I need to wait a bit and save for it, then so be it. The 15-85 has great IQ, but if you have the $700 the 15-85 costs I suggest just saving a bit more and getting the 17-55. It's a near-perfect lens: useful zoom range, great IQ, three-stop IS, 2.8 aperture. If there's any reason you can't afford the 17-55, the 15-85 should be next in line. You'll give up the f/2.8 aperture, though.


my 2¢


brendan