Tim - I think you're gonna have a blast with the new 15mm.
Robert - Thanks man! I meant to tell you how much I liked your MMA shots in your recent thread. You capture some really great emotions in those while shooting in real difficult lighting.
I'm certainly no expert nor Mark who's far more talented; I use the fish mostly for other purposes than Mark (whose approach of being super close to the subject I love, and it works well).
I usually use the fish when I want to convey the vastness of something. In a day and age when every child has a 16-35 or equivalent in their bag, the notion of "wide" often gets lost. I had a Sigma 12-24 (the 1st gen) and got rid of it not because it sucked optically (where it was so-so), but rather because it made conveying "wide" pretty hard. Yes, strong foreground/background is key, but realistically it can't be often achieved. That's when the fish comes in - the totally sick perspective, with the horizon always off center to stress the curve, and if the horizon is the actual horizon then ideally the horizon in the upper part of the frame to make it curved like the actual earth - further bringing home the point of scale. Defishing the image, even if I were to get something like a 10mm full frame view (not that such a thing exists), would be against the message that I'm trying to convey.
My best examples of this are from my past two years with NASA, and of course the lazy seal, my all-time favorite. In some of these images it would have been just as possible to get the shot with a 16-35 or certainly a 12-24 on FF. Also, in pretty much all of the images it would have been impossible to play the foreground/background game (except with the seal, where I am literally two or so inches away from the ice berg). I on purpose always chose the fish to drive a point home.