OK ... I'm rounding out my kit (350D + 17-40/4L + 50/1.8 II + 70-200/4L) with an ultra-wide lens of some sort and I'm stuck between the 15mm fisheye and the 10-22mm zoom. I really love the idea of having a 180-degree FOV, even if it's wildly distorted, and can think of a few upcoming trips where such a lens would be *really* handy to have. The 10-22 seems more verastile, costs $100 more, and is EF-S (which is a problem if I ever upgrade to a full-frame body). Also, a fixed f/2.8 on the 15mm is a huge gain over the f/3.5-5.6 on the 10-22mm. I think the biggest problem for me is that I'll be giving up a lot of the fisheye effect to the 1.6x crop on my Rebel (effectively turning 15mm into 24mm), but I don't know what effect that ultimately has. I'm totally stuck on this, so I'm turning to the photo geeks here at MacRumors to help me out. Comments? Suggestions?