If you're talking about a fisheye adapter - ie, something that goes over your existing lens to give it a fisheye look - they are, by all indications, total junk and not worth their cheap cost.
You need a proper fisheye lens to do it right. You can get the Zenitar lens from Russia via eBay for relatively cheap ($125-150), it comes in Pentax, M42, and Nikon mounts and there are adapters to use the M42 one on a Canon. It is 100% manual; if you want a fancier one with an automatic aperature and/or autofocus you'll need another one and are probably looking at a few hundreds dollars.
You'll also have to remember that a lens that gives a full fisheye effect on a 35mm SLR will lose some of that on a DSLR due to the edges being cut off. If you want the full 180' fish on a DSLR you need an even wider lens - something like 10mm or so.