I have used the Nikon 12-24mm several times (with thanks to Peter Rowlands and Robert Delfs) and I am debating whether to buy one for myself.

My concern is with the optical quality of this lens - does it really cut it. If Nikon were to bring out a 12-24mm with similar quality to the superb 17-35mm AFS I would buy it. But I doubt that this will happen. The 12-24mm is a similar quality to the 18-35mm.
At present I am using a 16mm FE as an alternative and like it very much. But I can't mount a UR Pro on it. Plus a wide rectilinear lens would be useful. Although I sometimes feel that the 12-24mm images have a tunnel look to my eyes! Maybe I shoot too much Fisheye?
Any thoughts, general comments or advice would be appreciated? And what about the alternatives from Sigma?
Alex