Sorry, I am running behind with reviews including some I'm writing for photo.net.
However I have looked briefly at the Tamron 70-300 and I can give you my initial impression that it is indeed a very good lens.
I'd say that at 300mm it's equal to the Canon 70-300/4-5.6 IS in the center and it's
better at the edges of the frame. At shorter focal lengths it's still good. At 70mm it may not be quite as sharp as the Canon, but they are close - and that's just a preliminary result which may change with further testing. Both are excellent.
The IS of the Tamrom works well, though I don't think it's quite as effective as the Canon IS. It's good at maybe 2-3 stops while the Canon may be good at 3-4 stops. Again, these numbers may change with further testing.
Overall my current impression is that the Tamron 70-300 is an excellent lens and great value at the current price of $399 (after mail-in rebate). As of now it's on my "recommended" list, and not too many 3rd party lenses are on there!
See
Tamron AF 70-300mm f/4.0-5.6 SP Di VC USD XLD Hope that's of some help. A full review will appear, but it may be another week or so (and with the holidays coming it might even be dealyed more, depending on how much free time I have).