Fisheye lenses

[Danosborne6661]

I just bought a Nikon D70 with a basic 17-70mm lense.
I'm interested in getting a fish eye for it, i don't want to fork out for a whole new lense but have seen you can get 'Add on' lenses that thread into where you would usually attach a filter.

Has anyone had experience with these? Obviously they won't be as good as the real thing but anyone have any comments on them?
Sample photos would be cool if you have any!

They're only about £40 or under for a .25x lense which look pretty extreme!