I too have looked at these, thinking it would be very cool to have. But it would be something I wouldn't shoot very often, at $2 a shot. I just can't, as of yet, justify spending several thousand on one, when I still need to get so many other guns for my collection.
I still need to get a .223 and probably .308/30-06, as well as a new shotgun, and so many other handguns. It will have to sit back there in my dreams until I start to run out of other things to buy.
I agree that a boat mount for it would be cool, if not a bit too extreme.