I've used the Sigma 12-14 on my old D300 body and really enjoyed it. It matches the sensor format of that camera and the edge softness has not been objectionable. I think it's more than offset by the creative avenues it opens up. This lens is just plain fun. Also makes a great interior lens when you want to cover a whole room while standing in the corner.
"He was a wise man who invented beer. -- Plato"