I'm getting used to the lens after buying it a couple of months back and taking it with me on one extended trip. But I also took old reliable 15mm fish which has long been one of my favorite lenses. It's not that I use it every day, but when I wants a fisheye, it's what I wants.
It is oddly enough a lens that can be used with great subtlety --or none. Optically, I'd say the new lens is better, but not to the point I'd worry about using the lighter single length 15mm. (even small amounts of weight sometimes matter to the old arthritic guy). It introduces a bit more in options with the variable zoom and it promises to let me use it with the 1.6 crops and my full frame 5D3s. It's a welcome addition.
As it happens, it's my third fisheye of the kit. I have an 8mm Russian manual focus lens, that does the full 180 or very close. Not one of my favorites but useful in the past. My working kit when I worked at this professionally included an 8mm Nikon for the Nikon F in 1963 or so. It paid for itself, the biggest problem was avoiding ad work for art directors who were on the wave with it -- something that happened after the late George Silk's sailing photography that including fish eyes afloat.