This lens has a great potential, being in 2012 the fastest available at 11 mm focal length (f/2.8). Quality is not perfect at full aperture; this is normal considering the average of the amateur lenses. My experience with this lens has been a nightmare but now, after having the 5th (!) sample, I got a reasonable quality lens.
Here are the main issues I've found on previous samples: 2 brand new from China cannot move the focusing ring from the A to M position. Fortunately, after 4 months, I succeed in getting back my money from China, but it was hard. A 3rd one was bought used in the meanwhile, but unfortunately it broke after a few weeks and exactly during my trip to Norway to image auroras (very bad!). I've payed (much) to get it repaired by official Tokina service, but the repaired lens was not able to focus at infinity. Sent back and then another new sample arrived (4th), but it was even worse! So another one (5th) arrived from Japan and finally I hope to use it to image the night sky.
From the first to the last sample, I've lost 8 months!!!
Be aware of this possibility when buying a Tokina lens.

I've tested on a Canon 350D (APS-C size). The behaviour is good when closing 1 stop, but at the corners the stars are never good. For a nice diffraction, i.e. Sun during the day, f/16 or f/22 should be used.
Even if this lens is designed only for small sensors, I've also tested on a 5D (full frame), at 16 mm focal (11 mm unusable due to severe vignetting). On 5D the behavior is bad at the corners and only acceptable at f/16 or better f/22.