I bought the tameron 17-50 f/2.8 hoping to save some money. Unfortunately, my copy had a major front focus problem. The color is superb (about the best of any of my lenses), and the IQ is good when the focus is on (for my copy that means either manual focus or leaning in after focus lock). Without pixel peeping, the shots look great!
Front focus and AF error in low light was an issue but I decided to do some comparisons before I sent mine back for calibration. Turns out even when the focus is perfect, the IQ is not quite up to my 70-200 f/2.8L or to the EF-S17-55 f/2.8 (which is to be expected given the copst difference). Some shots were comparable, especially shots of things with small text and lines, but for my "real world" shots of people, the Tameron just didn't capture all the detail in hair and eyelashes that the Canon lenses did. There was just more detail in the Canon shots (regardless of what the ISO charts show). At normal magnification and print sizes this may be undiscernable, but it's in the pixels. I have this thing about sharp eyes in pictures, bordering on obsessive.
In the end I decided to send the Tameron back and bite the bullet for the Canon ($150 in rebates/coupons made this a bit easier). Five years from now the cost won't even be on my mind, but having AF accuracy and high IQ right now will give me a higher shot percentage. Ultra quiet and faster USM; better low light AF; FTM, and IS are all perks that atleast justify some of the cost difference.
By the way, I tested the Promaster version, and it was pretty much dead-on with it's AF and seemed even sharper than my Tameron copy. That Promaster copy was a pretty good performer and if you don't demand that extra IQ, these lenses will give you some great photos.