I have an old Kiev metering prism that I use on a pentacon, and the meter is a lot more trouble than it is worth. First, it is not coupled to anything- you set the film speed, set the maximum aperture of the lens that is mounted, then spin a dial till the lights in the finder tell you it is right. You then read the exposure off the top of the meter and set the camera. If I have to take my eye away from the prism to set the exposure, what is the point?
My other gripe, and keep in mind mine is the really old style prism, is that it has a bad tendancy to get turned on and off coming and going from a bag, and the meter wheel gets knocked all about because it sticks out just looking for stuff to hang up on.
I like the prism; it makes the pentacon like a big 35 slr, it is bright, and I can focus well with it, but the meter sucks big time.