This lens should have an exceptional quality. Virtually no (rectilinear) distortion at 12mm (unlinke other 16mm wideangles), but there is vignetting.
I have a Sigma 10-20 on a Canon 7d which makes excellent wide field sky pics.
Only one catch: as these wide angles are rectilinear, distortion can be a problem with astrophotos even when rectilinear distortion is low.
I made for example this image with the 10-20 @ 10 (equivalent with 16mm on FF):
http://sky.velp.info/skypics2/IMG_6204.JPG
At 12mm distortion can be even worse than 16mm.
Maybe a fisheye suits better if this distortion is a problem.
On the edges the constellations look larger.
Test reports on the 12-24:
http://www.the-digital-picture.com/R...ns-Review.aspx
http://www.photozone.de/canon_eos_ff...a1224f4556iiff