I've never owned one but read various posts suggesting they are great frames considering their age. I've got my fave retro full Susser, a GT team RTS which I won't part with, love it although the rear shock is proprietary so aside from risse, no alternatives, which is similar to proflex in that respect.
Amp frames and variants like litespeed, mongoose are nice options but again, limited shock options.
What sort of bike are you after? I've got a not so retro Marin full Susser I bought for my eldest son, but at 13, it's too big so gathering dust. I may have an amp b4 to sell, haven't decided yet. Both are silver, the amp looks quite similar to the proflex.
I also have a litespeed full Susser, ti front end, amp rear I have had for years and never had the time to build. I really must, or sell.