I have and use a prism finder for my Mamiya C330, but only sometimes.
It is a well made and useful accessory that gives me a reasonably bright, non-reversed and upright image with the camera at eye-level. For portraits on the go (think weddings) it allows me to avoid the "navel eye" view of the world.
It is reasonably compact, but quite heavy. It is much more usable than the "porro" finder that Mamiya made for their TLRs, which are large, fairly dim, and give a reversed image.
Here it is in operation, with the camera, a 135mm lens, and me!