I haven't used a Mamiya 645, so I can't compare the out-of-focus rendering but I like the 55mm on my TLR. Perhaps not quite as good as the celebrated 180 Super, but just fine for my purposes.
I have all of the Mamiya TLR lenses and the 55mm is probably the one I use most of all. When I'm shooting with this lens I'm almost always aiming for max depth of field, but here are a couple of close up shots that show the bokeh...
The 55mm is a superb lens though I wouldn't normally choose it for its bokeh characteristics - I think there are better lenses for this purpose. The specimens I've used have always been sharp (stop down to f8) and quite contrasty - make sure you get the correct hood! This is a good lens to use in the great outdoors. Light and compact. Go for it.
I was looking for a wide-angle lens and the 55mm seems to fit the bill. I'm not trying to be a bokeh freak or anything, but I really dislike harsh bokeh, and the Mamiya 7 is known for that.
I was wondering if the 55mm on the TLR shared a similar reputation; if it did then I'd opt for a 645 camera like the ones offered from Mamiya, Pentax, Bronica, or maybe even Fuji. The 55mm on this camera seems worthwhile; although a bit slower than the 645 SLR's.