I originally thought my Tamron 17-50mm F/2.8 VC, which everyone claims is 95% as good as the 17-55 , could get me through whatever I encountered. Didn't happen. I purchased a Nikon 80-200mm F/2.8 two ring right before a wedding, and was blown away by the quality.
Super Sharp, great color and contrast; No contest vs Tamron 17-50 F/2.8 VC
Some people may be put off by the size/weight of a pro lens (not me :D