I was very surprised by this lens. It caught me way off guard. I've been very fond of Sigma lenses and getting a Tamron was kind of a new thing, but I'm glad I got it. For the price, I think I underpaid for this amazing find.
Wide open, the lens is great at 17mm. It's center sharpness is hands down sharper than my 18-55mm mkII. I couldn't believe my eyes. My hands got cuts from holding my camera. That's how sharp they were. Stopped down, It became a hell of a lot sharper. Corners were pretty "meh" wide open, but from f/5.6 and on ward it was fine.
When I got then lens, it didn't come with the lens hood but I bought a Canon EW-83E (the one for the Canon 17-40mm) and It fit perfectly. A bit tight, but I was a perfect match. Made my lens look more "professional" but at the same time, It was amazingly HUGE! If I were to take a photo with the camera facing forward, I wouldn't be able to see the camera.
Speaking of huge, this lens is pretty darn heavy compared to a standard kit lens. I was surprised at how light it looked. Even with my battery grip attached, the lens managed to out weigh my camera. It's not a major problem, but when shooting with a flash the camera needs way more support.
The focusing ring turns smoothly, and the zoom ring, although reversed) turns without any hicups. The focusing ring coupled with a follow focus would be a videographers dream. It's really smooth like cream.
I'm using it on a crop body so effectively my actual focal length is 27.2-56mm so it's replaced my Canon 18-55mm as my wide angle walk around. I'm not really missing the extra 20mm because I have the Canon EF 50mm f/1.8. It's pretty good for what it offers and I'm glad I made the jump.
If you are like me where money is a pretty large issue, and you're looking for a bargin-esque lens. Try finding this lens for APS-C sensors. It's a great kit lens replacement and It's a lens that can be brought if you're moving onto full frame