Overall the is a pretty good lens. It's sharp in the center wide open and still good out to the corners of a full frame image, especially if stopped down a stop or two. When compared with the Canon EF 24-105/4L, it has equal or higher image quality at all focal lengths and is significantly better at 24mm.
At around $1300, the price of the Tamron AF 24-70mm f/2.8 SP Di USD VC compares very favorably with that of the Nikon 24-70/2.8 ($1900) and Canon's new 24-70/2.8 II ($2300). Also, not only is it significantly less expensive, but it is also image stabilized which neither the Nikon nor Canon lenses are. This can significantly extend the low light capability of the lens, making handheld shooting indoors with available light and easier task.
Is it as good as the new ? I don't know since I have not yet had the chance to evaluate the Canon lens. I would not be surprised if the Canon lens was somewhat sharper wide open in the corners, but that's just a guess at this point. However even if it is sharper you have to ask the question whether any additional sharpness would be worth an extra $1000 and whether or not some of any additional sharpness might be lost in hand held situations in low light due to the lack of optical stabilization.
The isn't perfect. Distortion and vignetting at 24mm on a full frame camera are both quite noticeable, in fact vignetting is noticeable when the lens is used wide open at all focal lengths - though that's a fault that's common to all fast 24-70 zooms including those from Nikon And Canon. These problems are fairly easy to correct in post exposure processing. Corner sharpness isn't as good as center sharpness, especially wide open, but again that's pretty much the case for all fast zoom lenses in this range.
However, given it's overall good performance for a 24-70/2.8 zoom, its relatively low price, effective image stabilization and a 6 year warranty, the is certainly excellent value and a lens I can recommend.
If you want to try the new Canon EF 24-70/2.8L II USM, it can be pre-ordered from: