I quite enjoyed taking the lens out there ... despite having my feet in the picture most of the time ;-)
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Jokes aside, I wonder how useful a circular fisheye would be, anyway?
Personally I think a fisheye is an essential lens especially if you do weddings, for the church pictures and the dance floor at the party it's really awesome.
Not a single time hasn't the couple asked for prints of fisheye shots for their wedding although I don't take more than 20-30 shots with the fisheye.
I use diagonal fisheye however, by using a circular fisheye on a crop sensor.... How many times I didn't crop circular fisheye on full frame? Never, However it gives you more freedom cropping since the lens covers more than 1.6 crop area.
If I were to choose, I would go easily for diagonal fisheye.
I don't question the utility, nor the fun factor, of a diagonal fisheye, especially not since one had appeared in my household (and you can see many of these pictures in my Instagram). It's the circular variety that makes me wonder (at least for non-specialised use), though the 4/3 format probably makes life easier for it because it's "taller" and the circle of the useful image is larger in the frame.
As said circular fisheye gives you more flexibility cropping than a diagonal fisheye however, I prefer diagonal fisheye.
I just finished shooting a church ceremony (50th wedding anniversary for 10 couples), I used my samyang 8mm on 7D2 not on RP
So yes at least for me I don't see a practical utility for circular fisheye, but people like numbers so a 4mm would sell better than a 6.5mm, and that's how I bought the 8mm when I was a 5D user.