Every time I see a picture of that lens with it's front-element extending so far out, I keep thinking "expensive accident waiting to happen."
Cool lens, but I think I'd almost be afraid to use it.
Have you looked at the TS-E 17mm f/4L? The bulbous front element of the TS-E 17mm protrudes much further than the fisheye zoom, and the TS-E lens can't even take a hood. Plus, it's a few hundred dollars more expensive.
Yeah... I'm a proud owner of the TS-E 17mm, and I must admit everytime I take it out it's excitement... absolutely nothing to protect the front element, to the point you'll start overreacting when you see overcast in the horizon or when the wind picks up *lol*.
But seriously, it's quite an experience to use this lens, especially with pictures that does away with the barrel distortions w/o the need of post-processing.
Of course, it's not a "fast" lens for any means (for me, a setup with tripod and wired control is absolutely necessary), and the use of liveview for focusing takes a big crunch out of battery time. However, I actually found out I enjoy the time I spend fussing with the setup. It tells you again that photography shouldn't be something that's rushed and simply involves triggering the shutter release as many times as you can.
The bottom line - a lens is a lens, and the important thing is to use it (though price-wise, the TS-E 17mm is only half the cost of the 300 f2.8, and that lens is often seen at places when balls go flying in random directions...)