I don't know a lot about them, but I've met about 10 (one used to demo and train where I went for Obedience classes and the rest all happened to be at an outdoor dog show and Agility trial one summer) and loved all but one. They were so full of life and just so much fun. Reserved with new people at first, but either quickly warmed up or appeased strangers. One was severely shy, and her owner said that can be a big thing in the breed, but most of the others were not shy.
They were fun, crazy, driven, high energy little dogs. Loved to work with their owners and were super fun to watch run Agility. They were very fast and put so much effort into everything! Their owners had nothing but praise for their dogs and adored them to no end. They're supposed to be very, very quick learners and all of these dogs knew a ton of different things. Their owners said that they were fairly relaxed in the house, though. Not to the point of sleeping, but they kept themselves busy without making nuisances of themselves. I remember one saying though that her dog has to get out and do something absolutely every single day, otherwise he's a terror around the home, so that wasn't fun for her if she was really sick or something.
There did seem to have a little bit of dog aggression tendency, though. Most of the ones I met were totally fine, but there were a couple that were not and their owners said that the Mudi does not always get along with other dogs. It seemed to me to be almost brought on by a lot of unexpelled energy and a very tenacious (almost terrier like is what it reminded me of) attitude. They definitely have a lot of attitude.
They bark at everything and anything. Bark when they're happy, bark when they're working, bark because they heard *something* (even if it was nothing), barked because a leaf blew by... basically it seemed like they just barked to hear their own voices. And unfortunately for them, they do not have nice voices. It was almost little dog high pitched, but with a screechy bigger dog all mixed in one. Most were pretty ear piercing.
Anyway, that's my very limited knowledge of the breed! Too bad we don't have any Mudi owners around to tell us more.