Put Simply: Totally Awesome
The story picks up right after the 1st MGA, with Snake returning from his last mission, where he somehow deveoloped amnesia (probably something to do with the acid). When he lands, he is immediatly forced into another mission, where he has to take back another hidden warehouse thing controlled by another crazy guy with super-weapons. It's the same stuff that MGS fans have now come to expect from the story, and its told through good cut scenes and the good old CODEC dialoge.
Which now looks pretty damn good too. MGA2 improves on the grahics of the last, which makes the game have an almost cartooney look, which is surprisingly good looking.
Which you can also enjoy in 3D now. The game comes packaged with the Solid Eye, which is basically a cardboard box that you slip over the PSP to play in 3D. Its a pretty damn cool piece of gear, and there are even a few movies which you can watch, but play for more than 15 min. and you'll have some trouble seeing straight. So be careful with it.
Still, gameplay is awesome, and there are even more cards you can collect. 500 freaking cards are out there, and anyone who can collect them all is a freaking god. You can customize your deck for any situation, and no 2 battles play the same.
Which you will play against the same old legally blind soldiers, robots, and cameras as we've come to expect. Bosses are totally insane, ranging from a gigantic monkey with detacable arms to a crazy guy who walks on the ceiling. You will not get bored.
Simply put, MGA2 it one of the best strategy/card collection games out there for the PSP, and if you even passivly like MGS, stop reading this and go get this now.