Hits more than it misses
Everyone picks up a Metal Gear game and wants to know about the story. In terms of comparisons, it's the poorest in the post-'98 era. Of course, that isn't saying much to its detriment. The story's strong, well-done and has enough twists to keep Kojima fans interested and going. In fact, it did have some pretty neat touches...a psychic walking Snake through certain areas and giving him tips that way, etc.
The actual gameplay works well most of the time, and you'll pick up some interesting kinks along the way to help you through steathily, but you will occasionally feel the urge to say "Screw it", and just start shooting.
The combat itself feels similar to other turn-based combat games, with both the benefits of thinking your way through it, and the frustrations of occasionally having to helplessly watch yourself get pummelled. But it doesn't quite give you the payback that many other turn-based games do.
One particular sequence where, in the dark, you can't see characters if they shoot you and you're forced to navigate through a narrow corridor was extremely frustrating.
But, for the most part, things work and you'll enjoy the game. It's not one of the major entries for the PSP and somewhat waters down the franchise a bit, but it's a solid game and worth a good 10 hours of your time.