Ask Bitter Jeweler...he has one and really seems to like it.
I've heard that edge to edge sharpness isn't as good as the Canon, but if you are typically shooting wide open (or close to it), it's likely that your edges will be outside your DOF anyway, so it may not be a huge issue. If you are using it as a landscape lens, then it might be something to consider.
Sharpness looks pretty good in the centre though (from the samples I've seen).
I had been using a Tamron 17-50mm F2.8 as my primary lens, and it was a great lens for me. Sure, the focus wasn't as quiet or as fast as Canon USM, but for the price, it's a great lens. I'm also thinking of adding this 28-75mm as my next lens. I just wish it it was 24-70mm, as I think I'd like more on the wide end.
Another consideration is the Sigma 24-70mm F2.8. The older version is nice and affordable like the Tamron, but they have a newer version which is close to $1000...and at that price, you might as well go all the way for the Canon.
Ask Bitter Jeweler...he has one and really seems to like it.