I use mine never... because I don't have one.
This may seem a snarky response, but it actually says a lot. I had $3K burning a hole in my pocket at one point and I considered buying a fisheye... and didn't... because there are just so few uses for them. They're super cool, but not worth it unless you happen to be one of the oddball photographers who needs or wants one a lot... or you just have more money than you know what to do with.