There are always exceptions to common experience, but the general trend I've been seeing over the past decade is the Nikon F plain prisms are rapidly veering more into pretty baubles to put on a collector shelf than pleasantly usable pieces of active camera gear. My own experience "mirrors" that of Millard Marc Thomas above: it is very very difficult now to track down a Nikon F plain prism that doesn't exhibit at least the beginnings of center line separation and a few desilvering floater spots near the eyepiece. Finding one that is also cosmetically undamaged is even harder.
A few years ago, after three decades as a diehard Nikon F2 fanatic, I stumbled across an irresistibly priced, really clean F "Apollo" body with plain prism. Much to my dismay, the pretty looking prism shell was far from pretty inside. It had the most desilvering-ravaged glass I've ever looked thru: dim, filled with floaters, thick heavy black separation line down the middle. Thus began a intensive search for a replacement F prism that was cosmetically and optically clean. It took three years (and a level of eBay scouring I hope never to repeat) to track down a nice replacement prism, plus another complete F "Apollo" body with plain prism, Both cost me dearly, much more than I would have paid had this not turned into a ridiculous Moby Dick quest instead of an ordinary gear purchase.
My reward for all that effort and expense? Within a year, both of my pristine plain F prisms began their inevitable decline. The first suddenly developed a roof separation line at top and bottom, which is slowly growing toward the middle to complete itself. Perfectly usable, but disappointing. The second suddenly developed a cluster of little black floaters hovering mid-view: tell tale sign that the eyepiece packing foam is turning to tar, retracting, and pulling bits of silver off the glass. Again, still perfectly usable, but the clock is ticking and I expect it will be pretty bad five years from now. I gather from F2 plain prism fanciers that a similar trend is happening with those, but more slowly with less severe separation, and it is easier to find pristine plain F2 prisms (at a price. of course) vs optically perfect F prisms.
In over thirty years, I've never yet had any trace of desilvering spots or separation lines develop in any of my F2 meter prisms or Nikkormat prisms. I've also been fortunate with my two inherited F Photomic FTn bodys: meter issues, yes, but optics have remained perfect. While the F Photomic FTn is known to be somewhat vulnerable, many of them have an internal protection shield between foam and prism glass similar to the later F2 meter prisms. It isn't unusual to see a desilvered F Photomic prism, but they apparently aren't decaying on nearly the scale of the plain F prisms (esp the early F prisms with rectangular eyepiece).
