I've got a Tamron 17-50mm f/2.8 and love it. Apples to apples third party lenses seem descent to me. They're target market is those that can't afford or justify the expense of the equivalent OEM gear. I can't imagine anyone on an unlimited budget getting anything but Canon gear. Me on the other hand my budget is quite limited as of late and I may have to make do with some third party lenses. I'm trying to be very selective in whatI get.