If folks dont mind using an external flash indoors im sure the 15-85mm can take some very nice captures. Im also fairly confident the 15-85mm is a much better built lens then the Tamron 17-50 VC.
That's a good point about the flash. I am so happy with my 15-85 + Sigma flash that I am starting to seriously regret my Sigma 30mm purchase. I don't know if it is (a) my lack of skills shooting at large apertures (b) a Sigma focusing issue or (c) lack of IS on the Sigma, but my indoor shots with my 15-85 are so much better with the Canon. I wish the weather here wasn't so bad or I would try some wide open long distance shots with the 15-85 to see how the crops look. All of my long shots are at f/8 - f/14 and the crops are pretty sharp in all corners and the center, but that should be expected from any lens.