What do i look for in a fisheye lens?

I want to get a fisheye lens for my 16th birthday. I own a nikon d5000, I'm not looking for expensive if its possible.probably just around 300 at most. I need help with a list of things to look for when I go to purchase one.

Honestly, a fisheye lens is kind of like a new toy that sucks. Its fun get and play with, but after a while it ends up collecting dust.

I have never met another photographer that loves the fisheye.

Cedyk has it right. Used fisheye lenses are really cheap. People buy one, use it for two weeks, and that's the end of it. I carried one around for a year, and barely used it, but when I did use it, it won me some awards. I had a full compliment of lenses, so a fisheye was an accessory for me. Buy a more useful lens first, 35mm 1.8 AFS is a good choice. When you have every lens you could want, then you get a fisheye.