It's sort of a weird product, taking the world and basic plot of MGS4, shrinking it down into a shooting gallery, and then trying to make shooting work on a platform with no buttons, light-guns, or anything like that. But this isn't just a "tap the enemies to kill them" sort of game. You actually have an on-screen reticle, and you move that around by moving your finger on the screen. Then you tap the screen to fire. Lifting your finger off the screen lets Snake take cover behind objects, where he'll regain health. And the iPhone's two-fingered pinch and spread zooming controls are used to swap between a standard rifle and a sniper rifle.
You'll also earn Drebin points when you complete missions, which can be used to unlock 3D models, which the game implies you should use with the iPhone's built-in screenshot function to create lock screen wallpapers. From the main menu, you can select a page that displays some brief text about a lot of the previous Metal Gear games, as if you need some sort of back story to truly appreciate moving a reticle and tapping the screen. Still, it's quite a bit deeper than most iPhone games get.
Overall, the controls feel fine and properly built for easy use on the iPhone's touch screen. But, conceptually, it doesn't really do with the world of Metal Gear Solid justice. The story is relegated to quick bits of text between missions, and staring at the back of Old Snake's head the entire time as you plink off enemy troops and encounter bosses doesn't quite qualify as the "tactical espionage action" that series fans are probably after. It's an interesting attempt that's quite a bit different than the extremely simple games that currently dominate the platform... but on a platform like the iPhone, "simple" reigns for a reason. I'd probably still rather play WordFu or Live Poker when I have a little time to kill on the ride home.