Lens sold in late 2010.
In terms of resolution the lens is on a very high level. The center performance is excellent throughout the whole range. The borders are (naturally) quite a bit worse especially at f/4 but very decent from f/5.6 onward. Distortions are comparatively low for an ultra-wide lens and vignetting may only be an issue at 12mm @ f/4. Chromatic aberrations are a problem at harsh contrast transitions. Beyond CAs the lens doesn't really like contra light. Using the supplied hood is a good idea.
For a full review, visit http://www.photozone.de/canon-eos/272-tokina-af-12-24mm-f4-at-x-pro-dx-canon-lens-test-report--review