One of the bewildering things about the explosion of App Store gaming is the sheer variety of successful approaches to it: traditional mobile puzzlers and arcade games, indie experiments, full 3D ports, and games like Metal Gear Solid Touch that take the well-travelled road of licensed mobile gaming, cutting grand concepts down to bite-size.
One of the most exciting things about it is that the unique configuration of the iPhone and iPod Touch hardware - notably, its complete lack of buttons - forces everybody to start from scratch to an extent, no matter which of these routes they're taking. The conventional is simply impossible, and you could argue that the Apple platform is seeing a more complete revolution in videogame interface than the Wii or DS have.
On the face of it, Metal Gear Solid Touch is a cheap and irrelevant knock-off from the bad old days of mobile gaming. It transposes Hideo Kojima's demented PS3 stealth epic Metal Gear Solid 4: Guns of the Patriots into a disposable and short-lived shooting gallery, the most basic of arcade games. But while it's certainly not substantial, it's made with care, polish, and a keen sense of the strengths of the subject matter, the platform, and the input device - your finger.
It's not finished, though. What your GBP 3.49 / EUR 4.99 buys you is twelve missions, each a matter of minutes long (if that), that take you through the sights and boss fights of the first three acts of MGS4. The final two acts will be delivered in eight more missions via a free update in the months to come. This is the downside of simple and swift online distribution - it makes a kind of corporate procrastination possible - and it's a bit disappointing, although in this case, your enjoyment of such a brief experience will at least be spread out by having to wait for half of it.
In each mission, Old Snake takes cover behind a wall in the foreground while the usual gallery of balaclava'd goons and genetically-engineered super-soldiers pop up and take pot shots at him. You guide the crosshair to them with your finger - not directly, it can be done anywhere on the screen - and tap to fire. A certain number of enemy kills or a boss kill later, the mission's over.
A ring of colour counts down clockwise around each enemy, and when it turns from yellow to red, they fire back - and they always hit. Lifting your finger from the screen ducks Snake safe back behind cover (although walls can be destroyed by some heavy weaponry) and regenerates health slowly. Naturally, it also means you can't aim. It's a beautifully logical and well-balanced scheme, and combined with some dense enemy patterns and the clarity of the clock timers, it makes target selection more overtly strategic than in most games of this type.