Back in the day I was a huge MUD player, but that went by the wayside quite a while ago. I do not know about roleyplaying though: I thought serious roleplayers tended to play MUSHes, not MUDs, per se, though in popular parlance the former is basically a category of the latter. And then again, the sort of people who still play MUDs are probably a somewhat different sort. Last MUD I really played much was God Wars II: The Supernaturalis. And it has a pretty in depth combat system that requires a lot of attention, reflex, and typing.
There's still alot of MUD players, I have several friends that play LoL or FF in a window with their MUD client in the background. Achaea has around 400 players during peak hours.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.