Arm's a good MUD, but theres one major problem. There are about as many imms as players. If one of them doesn't like you youre screwed. cross one of them and your Arm career will never be the same.
Most of the good things that have been said about Arm in these reviews are accurate and true. But, like I said, slip up once and you're done. Every Imm, and there are a bunch of them - most who you won't even know - will come down on you hard and never forget it. You're expected to follow the Imm's lead while roleplaying, they like you to think they're always watching, and indeed often they'll animate someone for you to roleplay with if no players are close by, but if you do something that they didn't forsee, or if you try to think for yourself, you'll never live it down. better make a new account and start fresh.
So, if you want a good MUD, and you can conform, then Arm is the place for you. If you're a free thinker, you'll get beaten down quickly.