UPATE FOR vistors from http://www.dslr-forum.de found in the comments...
Occasionally one of the third party lens manufacturers comes up with a winning combination and this is one of those times that Tamron does so with it's latest 60th anniversary edition of the 70-300 F/4-5.6.
First off Tamron didn't hold back on anything with this lens. It's got their version of image stabilizing (Vibration Control) and their latest silent auto focus motor drive (Ultrasonic Drive). I had the occasion to compare this lens against the also spectacular new Canon EF 70-300 F/4-5.6L IS USM.
Both of these lenses are priced far apart from each other, but both offer different benefits from each other. If you're one that can't bring yourself to buying anything but a brand named lens then the Canon version will not disappoint.
Click to View original
Here's a pair of shots taken handheld. The Canon is much sharper but colour wise they both look pretty good. Both lenses control CA very well, but the Tamron is much softer and the VC motor is a little jittery. I actually found it disturbing, but I found the image stabilizer to be more stable on the Tamron than the Canon (this could be completely perception). The approaches are completely different by the two. Tamron tries really hard to hold the image stable then jumps a little to the next frame position. The Canon is more like a rubber band as it slowly bounces into the next frame position. Either way the performance is about equal.
Click to View original
These sample shows how well these lenses holds up as a close up lenses. These remaining shots are taken on a tripod and on the 7D with IS/VC off. The higher pixel density definitely shows the weaknesses of the Tamron lens compared to the Canon. The Tamron has some lens breathing going on so the scale is much larger. Good for Tamron. Performance wise, Tamron is much better as a close-up lens. There's still some purple fringing going on in the text but generally it's still pretty sharp.
Click to View Original
On this sample you can see where the Tamron is totally trumped by the Canon one. Hands down, contrast and sharpness blows the Tamron out of the water.
On the 5DmkII it's all a little more forgiving. Considering the pixel density of the 7D is so high, the Tamron fairs okay. I didn't get the chance to test the older non-L Canon but just based on perception I think the Tamron is still a better lens. Overall for the price, the Tamron is good value and is cheaper than the non L version of the Canon, but in close-up situations is much better than the L lens. I'm a macro and close-up shooter so I can appreciate how nice this lens is an up close performer.
I did not test them at 70mm so not all situations were covered, but from a 300mm perspective you can certainly see how they compare. One thing I want to add here, this comparison shows how micro adjust will not make a difference on zoom lenses. The Canon is softer than the Tamron up close @300mm, but far away the Canon is significantly sharper. Calibration of a lens is very complicated, and micro adjustment does nothing on a zoom lens with these kinds of tolerances. One thing that I'll say about the Tamron is that it is consistent from one distance to another. This might be a reasonable argument to choose the Tamron over the 3.25 times more expensive L lens.
The lightweight nature of this lens is probably the biggest seller of this lens. Combined with a low cost, I'd certainly still recommend it. Probably the most likely question is whether or not the L lens is worth the higher price tag. I'm not totally sure about that. If you're a L lens fan, then sure. The Tamron is not weather sealed, but has a Tri axial stabilizer (pitch, roll, yaw), while the Canon only has dual axial stabilization (only the new hybrid IS has three axial stabilization) Picture quality does tell a lot, but on the 5Dmk2 and in the close-up range, the Tamron appears to be an adequate if not equal winner.