I have a sliding EOS-mount back that attaches to a Graflok back on a 4x5" camera. It's interesting, but only in limited situations. Depending on the 4x5" camera, lensboard, etc., the widest lens you can use is going to be around 105mm, because of the mirror box in the DSLR and adapter tube, and the tube introduces some vignetting, so it's not as if you can stitch over the whole 4x5" field. If you use a mirrorless camera, you can stitch over a wider surface and possibly use wider lenses.
So if you're just using it in single shot mode, think of it as a very flexible alternative to something like a 90mm T-S lens, good for still life and tabletop work.
If you want to stitch, depending on the focal length of the lens, you might be able to shoot 4 or 6 overlapping images to make a 65mm square with a full-frame camera. Use artificial lighting, because any change in the light will reduce the ability to make a clean stitch. Again, it's not a bad solution for still life/tabletop work, particularly with strobes, if you need a really high-res image.
I've also tried doing this a bit with old soft-focus lenses, but stitching doesn't go well with portrait subjects. Still, you can get some interesting results with other kinds of subjects and these lenses. Here's one I did a while ago--
Spring in Soft Focus, Maspeth, Queens, New York | Flickr - Photo Sharing!