here is a pic with a fishy adapter:
you can see, pretty bad vignetting since it most likely wont cover the frame totally and chromatic aberration sets in quite a bit, mainly around the edges of the frame, degrading the image quality. I've seen samples of one attached to a 50mm 1.8 shot wide open and its just uggggh haha
as for a real fisheye:
pretty clear you can see the difference in sharpness, the image fills the frame and chromatic aberation is controlled fairly well around the edges...
hence the 900odd dollar difference in them ;p haha