My kids love Moshi Monsters! It's a great way for kids to start using the internet in a safe environment, and the fact that they have to do various puzzles to earn in-play money is far better then some of the mindless games out there.
I have been in contact with the Support team on a few occasions and they are great and quick to respond to any questions.
They use a lot of technology and monitoring to keep the whole experience child safe and friendly. The way kids communicate via their personal notice boards means I can keep an eye on things myself.
My oldest son has started using his spending money to buy his own membership, so now he's behaving around the house :)