don't think it's overlap
fisheye effect i think can PS, but need quite a bit of effort
the question is, do you really need a FE?
i mean, the effect is cool and all that
but it does get a little boring after a while
i would imagine that if i got a fe i won't really use it that much, so it'd realy be a very veyr spare spare lens
don't mind me posting an old pic taken by the 8mm?
basically what circular means exactly that, you get a circle for an image.. an extremely wide fov, think of a hemi-sphere.. whereas diagonal you get a "full-frame" image, where you get 180degrees fov along the diagonal..
I use the Tokina fisheye zoom. I got the fisheye over a wide angle zoom for my 30D cause I can always convert the fisheye to wide angle. Its actually quite easy, don't use the lens correction filter but use free transform, free warp. twitch the image a little here and there you'll get a wide angle.
You can duplicate an ultrawide by linearizing the fisheye shot in software, but you cannot do the reverse. Fisheyes have a diagonal field of view of 180 degrees - something physically impossible for a rectilinear (non-fisheye) lens. Also, after you convert a fisheye picture, the aspect ratio is different from the original, ie no longer 3:2; you'd need to crop some more.
i use a fisheye, as well as a Sigma 12-24 and the Canon 16-35. IMHO, the fisheye gives unique perspective that a normal wide angle cannot. i bought the fisheye on a whim, thinking i would sell it off soon; it has been with me for many years and has become one of my must-bring lenses for weddings.