I have been playind discworld MUD for a while now, and i am pleased to admit i am addicted. It is my first MUD, and my favourite.
The best thing about the MUD is the atmosphere. If ever a newbie needs advice, they can ask on the specialised newbie channel and obtain 10 or more responses. There are talker channels for each guild as well, and each guild bonds together to form a tight group (with some friendly rivalrys occouring between guilds due to this!)
The MUD is massive. I have been playing with various characters for a few months now, and I still haven't even scratched the surface of this involving MUD.
And now is the best time to start playing. As well as the established citys of Ankh-Morpork, Sto-lat, and Knot-Lip-Khin (thats right, _three_ cities), and a generous smattering of small towns and villages, a whole new continent has just opened up. The city of Bes Pelargic on the Counterweight Conternent has recently opened it's doors, allowing players more oportunities for exploring (and in my case dying!) in new and exciteing ways.
I could go on about the involving quests, the hilarious emote commands and the intracate combat experiences, but I'd better not- The Discworld is calling...