I had the Nikon 10.5mm DX fisheye for my old D70. When I upgraded to a D700, I never bothered to get another fisheye, but instead got a super-wideangle lens (the outrageous 14-24mm zoom).

Why?

A fisheye just changes the way you look at the world. It's exceptionally difficult to frame things well with a fisheye, but you do have the ability to get up close and personal with your subject. Also, in post-processing, you can straighten out a fisheye image, or even combine several of them, kinda like Android's new PhotoSphere thing.

For my newest camera, a Fuji X-Pro 1, I bought two prime lenses: a 35mm f/1.4 and an 18mm f/2.0 lens (equivalent to 52 and 27mm field-of-view, respectively). I find myself mostly just using the 35mm standard lens rather than the wide-angle lens.

That said, I'm pondering converting my old 10.5mm DX fisheye into a full-frame circular fisheye, although it's requires serious courage to do the process.