Advice on fisheye lenses

[Will T]

Hi all. I'm looking at adding a very basic wide-angle fisheye lens to my inventory (to be used with my trusty D70).

Trawling around Ebay, I notice there's a wide variety of lenses and prices in the fisheye category, ranging from $70-100 to around $800 for a sparkling new Nikkor. What sort of features would I be trading off at the lower end of the range? I'm an entry-level, infrequent DSLR photographer, and couldn't imagine using the lens too often. To that end, advanced features such as VR would certainly be overkill. But if I'm happy to contend with an all-manual lens, is there any reason I should avoid a $50-100 second hand one?

Does anyone here much experience with fisheye lenses generally? Any tips when using them?

Thanks in advance, Will