Also - should mention that the Tokina's build quality is also really nice. I haven't played with tamron as a 3rd party yet and sigma lenses that i've tried have felt cheap so far. So - I don't know if you're looking for a low cost route to L lenses (I am until I start pulling in enough income from the photography to pay for it : ) - the Tokina's seem to be pretty damn good.

(That and Tokina's optics are made by hoya - whose filters I've been a fan of)