Update 2020: Nikon has updated this fisheye lens with a newer much-improved model the Nikon 8-15mm fisheye; Link below. The new fisheye provides both a circular and diagonal fisheye effect in one lens using a full-frame sensor camera. That said, you should be able to pick this fine lens up used at a very reasonable price. Since the 16mm is so small it almost begs to be carried with you during your photography outings.
This lens is one of my favorites. It offers Full-frame, 180-degree fisheye coverage using Nikon's D-type autofocus lens technology. All I can say is I love this lens, and worth every prime penny I paid for it. Although I do not use it as often as other lenses the photos I do take with it I truly enjoy, some on this site use this lens. It is awesome. The lens is almost fish eye perfection if there is such a thing.
The biggest benefit for me is (1) the extremely wide field of view it can capture and (2) The Nikon 16mm lens is so small and light it almost always stays with me tucked away in my bag and (3) the lens focuses very close, giving you a macro like an effect. (4) The overall performance is excellent and due to the extreme coverage, fast aperture hand holding in low light is a snap.
I purchased this lens several years ago and today it still remains one of my favorites. As I have stated elsewhere this lens is light, and a great addition to your photo bag.
Sometimes I see people make comments like "why hasn't Nikon upgraded the lens"? I think the answer is especially in this case; Nikon has released an 8 - 16mm fisheye providing both a circular and diagonal lens with one fisheye zoom. However, this little 16mm lens is a small, light, and fast fisheye as you can get so for lower light conditions, this is a perfect choice.
I bought this gem years ago and the price now is higher - inflation I guess.
Overall I am very pleased with the purchase and the lens has performed extremely well over the years - and I now have the newer Nikon 8-16 fisheye zoom to add to my fisheye fetish.
Should you buy a fisheye? For those photographers starting out, I recommend you hold off on the fisheye and use your coins for another more practical day to day primes, then as you complete you need to have then bought a nice to have fisheye.
True Toad Recommends one for your bag!