I use this underwater and it is AMAZING. I used it to replace the fisheye 10-17 which I thought was pretty good until I tried using this.
It is sharp, with superb colours, and is one hell of a lens. Makes my previous 10-17 shots look flat and lifeless. No kidding. Cannot honestly say if the Nikon version is any better, but I do not see how it could be.
Built to last, with a clutch system that doesnt seem too problematic if you have two hands and a modicum of comon sense, this lens is everything you need for those wide shots, without the fishyness of the 10-17.