...how many times are you going to shoot a full circular fish-eye shot, which is essentially a round circle in black rectangle? Once, twice in your life? That's real useful.
You're saying you're surprised that anyone would find this lens somewhat limited on a full frame?
I'm saying it's more useful on FF than on 1.3x or 1.6x. "Most useful" as in relative to use on crop sensors, in that on the latter you only get full frame fisheye (and there other options for that on 1.6x), while on FF you get the option of either full frame or circular fisheye.
Now, before you go knocking the virtues of full frame or circular fisheye, take me out of the discussion. I was speaking in relative terms, pointing out that this fisheye zoom is more useful on FF than on crop. In absolute terms of utility, while I'm sure there are photographers who will love this lens, *personally* I think an extensive collection of navel lint has more utility than a fisheye zoom.