Got to say I disagree about the Zenitar MF 16mm/2.8 fisheye. I've got one and once you set it up properly and stop it down to f8-ish it produces lovely images. Out of the factory quality is apparently awful. Infinity focus is often not possible, the hood doesn't align with the body and there are often bits of metal rattling inside. But if you are careful and follow the tutorials on the web all of these can be fixed without too much hassle.
Most important thing is to use the filters provided. I bought one from a wedding tog who hated the soft images his copy produced. So did I, but a quick trawl on the web revealed that the filters which had been ignored and left in the box by both of us are part of the optical path of the lens. Pop the clear one in and the quality jumps up. OK, it isn't Canon/Nikon quality but the lens costs £100 (although see Ken Rockwell's review). Nobody looks at fisheye shots expecting edge-to-edge sharpness, they want a fun shot.