What should I look for when buying a Fisheye lens?

I've currently got a Nikon D3000, and I want a fish eye lens for it, what should i look for when buying one? What makes a good fish eye lens and what makes a *** one? Is there really that much difference between them? Anything i should stay away from?
That sort of thing really.

A bad fisheye lens is one that screws onto the front of a lens you already have attached to your camera and costs under $100. A good fisheye lens attaches directly to Tue camera body and costs at least $500.

Precision Design 0.25x Super AF Fisheye Lens is manufactured in Japan and have been engineered to do one thing and do it well; reduce the focal length of a lens by.25X to provide an amazing fish eye view. Depending on the camera model and lens it's mounted on, the image can cover the entire frame edge to edge or provide a circular image fully contained within the frame. Mounted to a zoom lens, the Precision Design.25X can often offer ultra wide angle, diagon