I played in -and enjoyed!- an RMSS game years ago. I never got past 3rd (?) level though, the game ended before I saw what a high level Rolemaster game was like. Low level was explore, run!, fight only when you had to, sometimes die from it, loot when you won. Not so very different from D&D, but it felt different at the time.
Play went surprisingly fast given the game's reputation, actually faster than my experience with D&D 3E (after leveling, and with tactical players) or 4E (ever). But this was with a GM who knew the system, who had us photocopy our common weapon and critical charts so we weren't flipping through the book, and who I believe was only using the two generic maneuver charts (also copied), not looking up the specific ones except on special occasions.
When you do that, play and combat are just roll, add mod off your character sheet, look up result. Which is, in principle, pretty fast and easy. I suppose the critique is, when enough users aren't doing that to give you your reputation, you've got a presentation problem at the very least.
So I feel it's a very under-rated system in that respect. Simultaneously, there are enough games out there its no longer as compelling an option for me. Especially when it looks very much like Rolemaster influenced the DNA of 3rd edition D&D (and therefore Pathfinder, and even Castles and Crusades). Between AD&D 2E and Rolemaster, I'll take Rolemaster. When it's C&C or Savage Worlds or ACKS or 3.x/Pathfinder or any of 20 other games or Rolemaster, RM hasn't gotten high enough up the list to come out of storage.