I've done some of this using the Mamiya 24 fisheye on my AFD/Phase camera, as well as quite a while back with my Canon fisheye. My preferred software for defishing AND general lens corrections is PTLens: http://www.epaperpress.com/ptlens/index.html
. Thomas wrote this very usable GUI which uses PanoTools as the guts, and IMO it is worth about 8 times the $25 he charges for it! I am currently using the Mac version and it runs very smoothly and fast on my system. The defish of the converted P45+ 16-bit tif (240MB total) shown below took about 15 seconds. The PTLens UI is realtime WYSIWYG as you make the adjustments and has a grid overlay for accuracy, and the final conversion maintains the original image dimensions.
Below is a quick example I shot in my atrium yesterday and just got around to processing today, so please excuse the uninteresting (and poorly lit) image as it is just for example purposes, containing lots of depth and lines to show how the image is affected. As you can see, the process is very viable. HOWEVER, lens anomalies will show up, especially in the corners where the pixels get stretched -- so minor CA can get significant in the corners. You lose about 10% of your focal length to the rectilinearization after the crop as well, but it is a cheap way to get a real wide shot if you only need them occasionally, and of course you get dual-duty from the fisheye lens to boot. In this example, the 24 fish turns into about a 26mm final, but frankly needs to be cropped in to about a 30mm FoV before the corners equal the Mamiya 28's performance.
The 30 Zeiss fish for the Hassy/Rollei may be significantly better than the Mamiya lens though, and given it's longer probably has less aberrations, so might be a good alternative to the widest rectilinear lens for your systems as you'll likely end up around a net of 34mm FoV or so.
At the end of the day, I am of the belief you are probably better served with a dedicated rectilinear 24 to 35 if you shoot wide a significant amount of the time, and of course here the digital-specific lenses from Schneider and Rodenstack will no doubt deliver a significantly superior result, but of course the cost of the added body and back adapter comes into play...