Well, I have to admit, I haven't played the realms for long, but I think I've seen enough to give a decent, albeit preliminary review for people looking for a mud.
Realms of Despair features a large, regular playerbase. The world it is set into is rather well written, with mediocre to good descs and a nice, well-rounded introduction for newbies. Levelling is slower than what I am used to, but that might be because of my inexperience.
What kind of surprised me with such a large mud was the lack of new concepts. Mind you, all is well done, but there's nothing much that sets it apart (technically) from stock muds. Personally, though, I think having lots of people around to play with makes up for that by far.
All in all: A robust MUD, if you want lots of people, certainly the place to visit - if you like something really innovative, go somewhere else.