I tried demon fish eye and one other, I cant think of the name. They both got returned after 10 seconds of using them. The main problem was the "adapter" they used. You would have to zoom almost all the way in to get the adapter out of frame. they basically made 2 lenses for every size lens. That wasnt the only problem though. The you could barely even tell it was fisheyed with them on. The glass wasnt powerful(?) enough. basically, the cheap lenses gave about a 1% fisheye look. They didnt do anything.
At NAB, I looked and Century optics stuff, and yes, they really worked. Of course they are proud of them. I think the quoted me around $700 for for one to put on my DSR. If you really want a good look, go for it.
Personally, if you arent gonna spend the money for an Century lens, I think the next best way to get the effect would be in post-production. I think a filter/plugin would do just a good job as a cheap lens.