The Canon 15mm FE is pretty nice, mine was so much better than the Samyang 14mm I bought it was like night and day. (The Samyang 14mmis not a fisheye, but it does distort everything)
I was able to pickup a used 15mm FE for $250, but deals like that don't happen often. The front cap was missing, but there were no marks on the glass, and a new cap cost $27. I used a front lens thread repair tool to expand the built-in metal hood very slightly so the cap fits snugly and securely.
Good luck in finding a decent one of these... getting exceptionally hard to find (in the Bay area, Cali) and when you do they are usually quite "used". Cost more for a used Canon than the Sigma... the Siggy is newer, cheaper (new or used), warrantied if new, sharper and better built. I chased the Canon 15mm for quite a while, finally tested out a few 8-15L but Siggy won out after all was said and done. I also tried a few selections in the Bower/Rokinon/Samyang route, 8mm / 14mm... meh, no AF, cheesy build. Sigma still kicked arse. Love their 10-20mm for a crop, a great choice for the 7D. Not a true fish on a the crop but still fishy enough.
Some rarely use their fishes, me... I use it quite often. I own a lot of "L" glass, actually all "L" except for the Sigma's I spoke of. When my son was about to arrive, it was this 15mm Fish that I took into the delivery room. There are lots of examples in my Flickr link if you want to see more...