It's in Beta right now, but seems to be running smoothly. Imagine Mechwarrior your typical mechwarrior in an online team death match. No missions so-to-speak, but there are objectives to try and complete in each round.
I've been embroiled in MWO since they opened the beta. It's a fantastic game. I'm also an old school Mechwarrior player btw.
It's a beta so the game environment is always changing and new mechs and maps appear almost monthly. It has full mech customization a la Mechwarrior 2 for both loadouts and weapons groups and is completely free to play. That being said if you want cool mechs and such you need time to build up money in-game to buy them or to fast forward by spending a little real cash.
Best mech game in a long time.