It's a pretty fun mode, but seems very easy. I'm hoping the advanced difficulty missions start appearing soon. I'm not sure it'll have the lasting replay value of standard TF2, but it's a nice alternative.
For those wanting to connect quickly, use the server browser and search for: Secure, not password protected, has users playing, is not full, australia, map "mvm" and then sort by number of players. As long as you don't pick ones which have 6+ players already then you should get in fine.