METAL GEAR SOLID: Peace Walker is not a game, it's a manifesto. Creator Hideo Kojima is so good at expressing his ideas through games that it's easier for him to make a point with a virtual allegory than a handwritten note.
Kojima is at his best when he's against something, whether it is the dangers in his games (nuclear proliferation, child soldiers, stupid codenames), or the fanbase that buys them. Metal Gear Solid 2, for instance, reduced hero Solid Snake to a glorified cameo in favor of a whiny teenager—and Kojima, with a horde of enraged fans at his heels, laughed all the way to the bank.
With Peace Walker, it seems that Kojima is raging against Metal Gear Solid 4's critics. He was widely slammed for filling MGS4 with cutscenes, so Peace Walker takes the opposite approach. Where MGS4 gave you 10 hours of gameplay and 40 hours of cutscenes, Peace Walker offers 50 to 100 hours of gameplay, and what cinematics do occur often include interactive segments where you have to quickly click a button to dodge a missile or shoot a robot.
In response to critics who claim that MGS4's gameplay was largely recycled, Peace Walker comes packed with an obscene amount of contrasting gameplay elements: You can raise a rebel army, manufacture guns, build your own Metal Gear walker, or chat up the local UFO nut about chupacabras. It's impressive that an action game includes 100 hours of gameplay, but it's more impressive that almost none of that time span is repetitive drudgery.
If you're a fan of the series, though, you're here for the stealth gameplay. Without discounting its PSP origins, Peace Walker's control scheme is as competent as any game in the series and it ranks second behind only MGS3 for pure gameplay quality. Thanks to its fantastic visuals, Peace Walker is the best-looking game on the PSP—and thanks to Kojima's rage at his critics, it's the best-playing game on Sony's handheld as well.