So I've been playing the Mag Beta and the Uncharted 2 demo recently. They are both pretty fun, and since Uncharted 2 is a demo and anyone can get it I'm going to focus on Mag.
If you've played Resistance 2 online and gotten into a skirmish you are going to know what to expect from Mag. Though I think Mag is done a little bit better, if only because the directives you have in Mag (in the 1 game type I've played) will actually help your cause, unlike Resistance, where they were designed to bring combatants together. In the beta you are part of an offensive team or a defensive team with the goal of defending (or capturing) 2 prototype vehicles from your enemy. Your particular squadron is tasked with various thinks like taking out bridges, defending bunkers, bringing down AA guns. And each thing you do helps. If you take out a bunker, for example, your enemy loses a spawning point near the front lines, and bunkers have a heavy machine gun on top that's great for defense. If you take out the AA gun you can parachute in as your spawn. Kind of a double edged sword since you can be shot on your way in, but if you land you're way behind enemy lines.
The game plays really well, controls are fairly standard, overall it's a blast and if you enjoy any FPS at all then you should enjoy MAG.