My mate Ed is selling his 70-200 f/2.8L, and graciously *cough* cheap sales pitch to try and sell it to me *cough* lent it to me to try out over the weekend. So here's a quick comparison between the Tamron and Canon.
The Tamron will set you back about AU$1000-1100 new, best price, where the Canon will cost more like $1500 for a grey import, and increasingly upwards for Australian stock. So there is definitely a premium to be paid for the Canon, but is it worth it?
I like the look of the Tamron more, the all black on the black camera, just looks right, it looks mean, and it gets comments. The strange white striped L lens, it looks the part to people who KNOW that "L lenses are white", so it's a show-off tool amongst other 'togs.
The lens cap is tough to get off on the Canon when the lens hood is on. The Tamron is fairly easy to get off when the hood is on.
Tamron has a pull/push AF/MF focus ring, which is a bit clunky, where the Canon has full time manual focusing and USM which is just easier, quieter, and faster.
The Tamron being a macro lens has a MFD of 0.93M (about 3 feet), where the Canon is 1.5M. That means the difference between a shot of a bug on a leaf, versus a shot of a leaf.
There is also a difference at the 200mm end, with the Tamron "zooming in" a little more, which is nice, giving you a bit more zoom, and the MFD makes it a bit more versatile.
If I had to choose from scratch, given that I already have a macro lens, I'd probably choose the Canon. Now that I have the Tamron, I think I'll wait and eventually go for the Canon IS model.