SO. Either I buy a used, "Perfect Condition" Canon 17-55 f/2.8 IS today for $800 even, or I buy the brand new Tamron 17-50 f/2.8 for $499 from Nat Cam. I am leaning towards the Canon, provided that it is a sharp copy with no focus issues, as I have heard that the IQ surpasses even the famed 24-70L, depending on copy. On top of that, the IS is a feature that I adore. On top of that, its $400 off current market price. However, after reading reviews, the Tamron seems to be fantastic for the price. I am planning on using it on my xTi (Soon to be 50 or 40D, waiting for a deal) as my main walk-around lens. I also plan on using it alongside my 50 f/1.8 for available light concert photos. Any personal experiences with the two? Any thoughts? Regrets? Thanks guys!