Earth Vs Moon is a fast-paced, well-made, and often funny arcade game that bears a close resemblance to the Atari classic Missile Command, along with quite a few homages to other beloved old-school video games.
The setup is similar to Missile Command: three stationary bases along the bottom of your screen that fire missiles up at incoming targets with a limited ammo supply. Earth Vs Moon makes great use of the touch screen (you tap to target), offering an improvement over the original game's trackball controls and the Atari 2600's joystick. Many of the game's levels play much like the original, as you race to predict the trajectory of multiple incoming missiles, but just as often Earth Vs Moon mixes things up with unusual challenges that borrow from arcade standbys like Sinistar and Space Invaders. Earth Vs Moon adds to that with slick, colorful graphics, touch-screen innovations (like a swipe to send a salvo), a system of perks (basically, purchase-able power-ups, e.g. "Death Blossom... Believe in yourself, Alex!"), and a detailed and funny backstory involving Moon Marshall Rats, Emperor Noom, and countless details like Moontanamo Bay and Earth's "Missile Commander" (that's you).
Earth Vs Moon definitely falls into the solidly good, worth-a-few-bucks camp of iPhone and iPod Touch games, and fortunately a new lite version was released so you can get a sense of the gameplay for free. If you can get the full version of Earth Vs Moon for free or at a discount, this is a must download.