If you're a fan of the Resident Evil games then make sure you check out Resident Evil: Degeneration Lite.
This iPhone version of the popular zombie slasher is a completely adventure where you must save the World from a mutant attack. The back story is that a lone zombie is released into an airport when a plane mysteriously crashes into the terminal. Soon, people are becoming infected left, right, and center - and it's your job to blow them to pieces.
Of course, the back story is never really that important in these kind of games, and above all, Resident Evil: Degeneration Lite is just a cracking portable version of a timeless classic. Controls in the game have been optimized for the iPhone. There's a touch analog stick on the bottom left for moving around and aiming. There's also a button for flicking between walking and shooting mode, an action button, and a button for changing weapons. Moving around takes a little getting used to if you're used to the dual analog stick versions of Resident Evil, but shooting is fairly straightforward.
Graphically, Resident Evil: Degeneration Lite is superbly executed. The animation is slick, the cut scenes are frighteningly realistic, and the sound effects are as haunting as any Resident Evil game I've played. The menu system is well laid-out, with clear, easy-to-use map and inventory screens.
One of the things that might annoy you about Resident Evil: Degeneration Lite is that you sometimes feel a little detached from the action. Many controls and actions are performed semi-automatically, and there are a few too many interruptions from your colleague sending you messages.
Overall though, Capcom does an impressive job of porting the Resident Evil format to the iPhone.