I wonder how many Tamron 24-70mm VC owners have ever used (for extended periods of time) the Canon II lens.
I used the Tamron 24-70mm f/2.8 VC for two years at a major studio, I ordered it for them based on raving reviews. What a giant hassle. That warranty is meaningless with a 7 month turnaround time on repairs (could have been a parts supply issue at the time, front element popped right off, was held in by glue, Roger Cicala covered this issue in depth). Still couldn't focus worth a damn in low-light when we got it back. Unless you try multiple copies, don't even bother. What's sharpness and IS matter if the image isn't in focus or you have to manually focus?
Also, it wasn't that sharp (had to stop down to f/8). Maybe we got a really bad copy. Try multiple copies instead of trusting reviews on it.
The 24-70mm f/2.8L II is a wedding/photojournalist's professional tool. It is the most boring, most reliable lens in my toolkit now. If you want a lens that you will use around the clock every day for 10 years, get the Canon II. If you want a lens to bring out for vacations and manually focus at night with, get the Tamron.
I wasn't going to bring it up, but build quality was one of my concerns with the Tamron too (based on some experience). Sample variation with any lens is an issue - though I believe Canon's recent lenses (24-70II, 100-400II, 16-36 f/4 IS, 35L II) have been very good in terms of their internal build quality and design (blogs from Cicala again).
I don't particularly find the 24-70II "boring", but I do think that, like a reliable car, you can end up taking it for granted; because it doesn't have any "interesting" foibles, and just gets the job done.

In all seriousness, though, I have an ancient 24-70 mk I and in spite of its known design defects, it just keeps chugging on, taking great shots. I shoot sports at least twice weekly with mine, and I can't complain about the results. I will buy a 24-70 mk II when this one fails or becomes unrepairable, because I know Canon has improved the optics and the mechanical construction - and it should be readily available for under US$1500 very soon. That's not much more than I paid for my used mk I.
. A nice super fast prime is desirable, but yea, it's a workhorse that just gets the job done. Personally I love it - though as already discussed on this thread; it's not the right tool for every job in the 24-70mm focal length range.


