Fisheye lenses are small and light, so they are easy to travel with. Judicious and selective use of the fisheye can yield results that perhaps could not be obtained any other way.
Generally they are excellent in situations where the distortion caused by lens either adds to the composition or where the distortion will not be noticeable. I like to use the distortion looking through columns to help frame an image like the pavilion shot at the Udai Villas, Udaipur Rajasthan India.
The 15mm fisheye is excellent for underwater photography, since usually there is not too much geometric structure worry about. However you have to get extremely close to the subject, to get a good shot with a fisheye underwater. In general something like 2 or 3 feet at most, should work well. Fortunately the 15mm fisheye has a good depth of field making it easier to get all of the subject in focus.
There are only two problems with the 15mm fisheye, firstly the lens cap is pretty sketchy since it does not positively attach to the lens. So it always feels as though it is about to fall off and expose the bulging and vulnerable lens surface. The other less serious issue is that the build quality seems a little plastic like. Having said that my fisheye lens is still in mint condition after years of use.
Addressing the relatively minor quibbles I had about the 15mm fisheye is a new fisheye zoom lens 8-15mm F/4.0. However more about that lens in a later blog.