They are useful for those times when you absolutely want the [subject][1] to know you are taking a picture, unlike a telephoto where you can shoot anonymously. ;-)

[1]: http://www.flickr.com/photos/rberteig/4272097014/

Seriously, fisheyes have a number of real applications:

 * They allow coverage of large fields of view, with a distortion that has a fairly simple mathematical model. They were invented for research imagery of the whole sky for measurement of cloud coverage (see [wikipedia][2]). Since the distortions are easily modeled, it is possible to make a quantitative measurement of the sky coverage from analysis of single exposures.

[2]: http://en.wikipedia.org/wiki/Fisheye_lens

 * If used with a tripod or similar stable mounting point and rotated about the effective entrance pupil, one can capture a full sphere panorama with only two exposures. Fewer exposures allows the panorama to be captured faster, and more than one exposure makes it possible for the photographer and their gear to not be in the finished picture.

 * They allow documenting a confined space with very limited incursion, often just barely leaning in the window is sufficient.

 * They allow coverage of large, sweeping landscapes without resorting to panorama adapters and stitching.

 * They don't really require much aim, so they can be used for stealthy or automated captures where it is more important to catch some kind of image of the subject at all, than it is to have an artistic image.

I carried a borrowed NIKKOR 10.5mm f/2.8G ED on a recent tour of Australian tourist spots. It stayed on the D90 for much more of the trip than I expected as I found opportunities to take pictures that would not have been possible with a more "normal" lens. I'd strongly suggest borrowing or renting on and trying it out on a casual outing. You might be surprised at the results.