I own a Canon 7D, and after owning a disappointing and extremely unreliable Sigma 24-70mm f/2.8, I decided to make a change. I wanted a fast Wide-Medium Telephoto Zoom lens that was pin sharp at f/2.8 throughout the focal range. The Sigma I owned was useless until f/4, which detracted from the reason i bought it in the first place. So after thorough investigation I narrowed my choice (due to budget) down to the Tamron 17-50, Sigma 18-50 and the new Tamron 17-50 VC. Cut a long story short, I decided to take my chance with the Tamron with VC. I already own the 18-270 VC, and know how good the VC works. A lot of Internet and Magazine reviews say that the Tamron 17-50 VC is soft wide open, and in the corners. They also say that the non VC model is much sharper and has more contrast. Well, sorry to buck the trend, but my copy is absolutely astonishing! I have corner to corner sharpness, and it is tack sharp at 2.8 wide, and zoomed in!!!
But here is the catch, upon opening the box I did some tests at home to see how good it was, and to be honest, it was OK but not great.......BUT the new line of Canon (and Nikon i believe) have a very handy tool in the menu. It's called AF Micro Adjustment. This tool allows you to calibrate each lens that you own, so that your AF is ALWAYS accurate. It's almost like fine tuning a TV, and it works an absolute treat. My Tamron seemed to be Front Focusing ever so slightly, but once i fine tuned it with that Menu......Oh my! The great thing is, once tuned, your body will remember it any time it is attached!! How cool is that!?!
So my Advice would be, if you have a Canon or Nikon, check to see if you have the calibration option in your menu. If you do........GET THIS LENS NOW!!! Once it is tuned, it'll give the Canon/Nikon equivalent a heck of a run for it's money......Trust me on that!!