Pretty much answered my own question after reading lots of discussion threads at dpreview.com, reviews on the B&H site, etc., etc. Most of the comments boil down to "it's very good for what it is," but that there are inevitable compromises with this great a zoom range such as softness anywhere except a few sweet spots. Makes sense for those who find lens changing a huge hassle, but otherwise not worth it. I wouldn't put my lens changing hassle that high on the pain scale.

A few people also reported compatibility issues with some Canon bodies. Canon naturally washed its hands of any responsibility, and Tamron seems slow to address them.

Anyhow, no sale.