Being such a HUGE Canon Fan, I was reluctant to buy from anyone else, although I have been happy with Sigma in the past.
This is the first Tamron lens I have owned- I bought it when I went to Haiti because I wanted a great lens with tremendous focal range (18-270mm is about as crazy as you can get). I was considering the Canon 18-200, but wanted the extra reach. Keep in mind your aperture will change (3.5-6.3) with your focal length, so this is best used as an outdoor lens with plenty of light.
Nearly all of the 7D images I took down there used this lens. I am very happy with it. When I purchased it here in Tempe, it was about $700, with the rebate Tamron just announced, you can get it for just over $500. In my book, its the most versatile walk-around lens available.
Did I mention Tamron has an 6 year warranty on it? Its been winning awards like crazy too! If you have any doubts, read the reviews on BhPhotos website:
Tamron 18-270mm 3.5-6.3