As popular as the iPhone is as a gaming platform, its greatest weakness is also what makes it such a versatile device: that touchscreen. Games above all other applications benefit from physical buttons and joysticks. And a joystick is just what this adds to the phone.
We've seen a joystick for the iPad that functioned along these lines, but this appears to be the first one small enough to work on the iPhone without blocking half the screen. Essentially, it's a little nub that will act as your finger when you move the joystick around. You can stick it anywhere on the screen, so just plop it down on top of the on-screen controls and it should do the rest.
It won't work for every game, naturally, but for arcade-style games like Pac-Man, it should do the trick. Is it worth $18? Well, that's for you to decide, friend.