An amazing game that pushes the boundaries of the system's hardware. The best stealth game on the PSP.
Even though the PSP doesn't have a second analog nub, the game controls perfectly. You use the face buttons to move the camera, R to attack, L to aim, move with the analog nub and do actions such as crouch and interact with the D-Pad. The game has the camouflage system from Metal Gear Solid 3: Snake Eater, although you can't change your camo mid-mission. Pressing against a wall or lying down increases the camo index, making you harder to spot, but you can't move while doing those.
The graphics look phenomenal for the PSP and the visuals are even comparable to some console games. Just for an example, you can see each hair in Snake's beard. Unlike a lot of PSP games, there aren't any texture seams either. The cutscenes resemble comic books and they look great.
Just like everything else in the game, the sound is amazing. You can hear bird calls in the jungle, the voice acting is excellent and the theme song 'Heavens Divide' is really good. Especially because it plays during an epic battle in the later missions.
You can do Extra Ops to gain GMP (the game's currency) and later on Outer Ops become available which let you send teams of your soldiers on missions to other countries. See, in Peace Walker you have to recruit soldiers, and it isn't that annoying drag-them-to-the-cardboard-box procedure that it was in Portable Ops. You can use the Fulton Recovery balloon to recover soldiers and carry on with your mission.
Once you defeat certain bosses, you can take them apart from the inside in a limited time and scavenge as many parts as you can, and you can use these to build your own Metal Gear to send in Outer Ops.
There's a whole lot of stuff to do in Peace Walker, and there are the Extra Ops to do once you've finished the story. It's the best stealth action game on the PSP and is recommended to anyone owning a PSP.