If you're a hardcore Metal Gear classicist, steer clear. Otherwise, fun as heck.
However!
This is not a "Tactical Espionage Action" game! Don't buy it expecting Metal Gear or Metal Gear Solid on PSP - it's a whole different game. The graphics are really the only flaw in the game - the sheer number of cards available makes for tremendous replayability - I'm 36 hours in and on a third playthrough trying to collect all the cards. The bosses are fairly and tough and very fun, and the multiplayer mode is as satisfying as any game on any other console. So if you love Metal Gear, but are looking for a break from stealth, check out Metal Gear AC!D 2.