The tamron 17-50 2.8 is about $420 and a lot of people swear by it. The non-vibration control version is supposed to be sharper. Is the Canon 17-55 IS better? Prolly, but at $900 for a refurb it better be. As far as 3rd party stuff goes, Sigma has a quality control issue but if you get a good one, it's great. Tokina has one in this range and is built solid, but has issues with CA that could be removed thru post processing, but...I'd stick with the Tamron and the constant 2.8 is a big plus.