I have the fisheye 15/2.8, and I must say this lens is extremely good value. Very sharp, wide (on FF), small, light, well-built, and fast (2.8 ). The downside is the metallic front cover which doesn't stay put, and the non-USM AF, but then again, AF is not that important on a fisheye (I mostly use MF on this lens anyway).
One of my favourite uses for the 15/2.8 is northern lights and astrophotography, and for those applications the aperture is very important. The new fisheye doesn't quite fill the same niche, so it's a bit unfortunate that the 15/2.8 is discontinued. It's understandable, however, considering the old design. I would suggest you get one while you can, or try to get a used one. If you are interested in fisheye on FF, this one will not disappoint.