I have passing familiarity. A former client used an MCP-based system (first an A Series, then later a Clearpath system on PC hardware) to run their line-of-business system (a small bank). A friend of mine worked for Unisys right out of college in the early 70's writing ALGOL code for the MCP. She says that it has its roots in the old Burroughs B5000. From what I've read they are odd beasts, and have some really interesting hardware-level security architecture. I never got to play with the system, aside from typing a couple of commands now-and-again into the OS "shell" (called MARC-- Menu Assisted something-or-other), and a little bit of editing in the editor CANDE.