It's very unstable, otherwise I'd be playing it right now myself. The Robotics class has very good area-denial with their deployable turrets and shields, and I had loads of fun with that before it started crashing constantly on me.
The story quests for the time being are limited to just the first zone, which'll get you up to about level 18 or so and load you out with all the starting armor you need to get into the pvp action.
Otherwise, this is more or less like Halo, where everyone just queues up and dukes it out with each other, albeit with four distinct playstyles, instead of 'who can grab the Spartan Laser first'. It's a lot of fun, provided it can run on your machine - write off anyone who has a FAT32 hard-drive, because you'll get nothing out of the actual game client, and be forced to use the Steam downloader to launch it. This makes things fairly well laggy, at times - especially, god forbid, if you hit shift+tab by accident.