I have a prism for my Mamiya C330. It, and the left hand grip were essential for my wedding work - the higher viewpoint is preferable for a lot of on-the-fly portrait work.
It is heavy and a bit large though, and definitely less bright than the waist level finder.
Nowadays I use the prism more as an accessory. I would guess it is on the camera about a 1/3 of the time.
Do not go for the porro-finder - it is a very poor substitute.