Tamron AF 70-300mm f/4.0-5.6 SP Di VC USD XLD...the bottom line is quality.

I owned and used this Tamron lens for 2 years on a Canon T4i and 70D. It compares favorably to Canon's 70-300mm L lens on the 70mm wide side, starting to lose @ 200mm and lose badly on the 300mm long side. It's IQ @300mm is only good if you step down the aperture to f11 and best @ f13. But for $1000 less, the Tamron is still a STEAL.

See for yourselves in the comparison charts on the-digital-picture.com site:

http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=757&Camera=453&Sample=0&FLI=0&API=0&LensComp=738&CameraComp=0&FLIComp=0&APIComp=0

I also found a secret to make the Tamron's IQ improve quite a bit at the 300mm long end: I use APS-C cropped sensor cameras only. I figured since the Tamron was built for full-frame cameras, when used on a cropped sensor camera, the front glass is really too big. Bigger glass means more imperfections from the outer rims of the glass. So I screwed multiple step-down rings to the filter thread and left a hole of about 35 or 34mm making the front glass exposure smaller, way smaller.

WHOALA, the IQ of the Tamron @ 300mm becomes very respectable @ f5.6. AF is still good and no vignette at all even when zoomed out to 70mm. I think that reduces the f-stop by a little bit making it like f6.5 (I'm guessing here), but who cares as long as AF still works.

Try it guys, you'll get an L lens equivalent IQ from this Tamron throughout the zoom range. Use the lens hood to cover the strange looking step-down rings and nobody will notice.  Mind you this only works on cropped sensor cameras. Full frame camera will vignette a lot, I imagine.