Samyang has a much cheaper fish eye for crop sensors that's a lot of fun to play with. It's fully manual, but it's not exactly hard to focus a lens with that kind of DOF.
There're lots of fisheye options out there. What sets this one apart is that it's a full-frame zoom that ranges from 180° full-hemisphere coverage to the standard 180° diagonal coverage. And, of course, that it's a Canon L lens.
If you were wanting both an 8 mm and a 15 mm fisheye lens for full frame, then your only other option at B&H would be the pair of Sigmas. The Sigma 8mm is $900, and the Sigma 15mm is $600...for a combined price of $150 more than the Canon zoom.
Of course, if you were only looking for the one or the other focal length, you could save quite a bit of money by just getting that one -- and there're a lot of inexpensive options for APS-C cameras, as well (including a bunch of ~$300 8mm f/3.5 lenses and a $700 Tokina 10-17 zoom).