I'm using MV on two projects: one for an National Science Foundation research project into MMOs and education; the other is creating a class for DeVry Online about teaching MMO skills to students.

So far it seems okay. It's very close to Second Life in terms of "feel". The business model is very attractive for education and research since you don't owe them anything unless you charge something for your services first. Their architecture is meant to appease all the goodies like server load balance, dynamic bandwidth throttling, etc.

Where did you find the 150 client statistic? To my knowledge, they hadn't taken it up beyond maybe 50 but that may have changed.

All in all, MV seems to be the leading contender for the highly prized title of MMO-Maker. It is backd by some HEAVY hollywood hitters as advisors (landau and cameron) and I have been personally told by the developers that they have enough of a cash "cushion" to burn through a few years before they have to start making it back.

This is DEFINITELY one to keep an eye on for the near furture.

(PS: it's free to become a developer and DL version 1.0 so if curious, nothing stops you from trying!)