The best free game available on XBLA...
The game only has about 6 levels overall, each one harder than the last. You take control of a ship and, simply, shoot everything in sight until you reach the end of the stage, fighting the occasional boss as well. You have your normal gun but you can also pick up more weapons along the way to help you like rockets, a laser, a "black hole", and plenty more. There's also an expert mode, where you have less lives and only one hit. That's pretty much it, just a simple game that serves as nothing but a nice time waster. Aegis Wing is pretty fun though, it's kind of like an old 2-D shooter, similar to Defender for the Atari.
The game is even more fun if you play 4 player co-op, online or off. The game is very hard by yourself so you'll have a better chance with friends helping you out. The one major difference in co-op is you can attach your ships together and increase your fire power. Pretty cool, but you have to rely on your partner to steer clear of enemy fire if he's controlling.
Aegis Wing is very simple, but it's not bad overall, especially since it's free. If you haven't downloaded it yet, you might as well since you have nothing to lose. Worth playing.