SNK's slowly been releasing all of its great old Metal Slug games from the Neo Geo library, and just recently, it worked its way up to Metal Slug X, which I think is the fourth iteration of the series? It's hard to keep track -- there's like 20 of them out there in history.
At any rate, if you've ever played a Metal Slug game before, you know what to expect here: You control a soldier that runs on a constantly moving 2D battlefield, firing away at enemies and dodging all sorts of colorful incoming fire. The Metal Slug games are known for their high-quality 2D animation, and this game is no different at all -- the sprites are extremely complicated, well-designed, and while the action gets frenetic, it's extremely well-drawn and produced.
This is actually an updated version of the game on iOS, with new bosses, new weapons, full Game Center integration and even multiplayer over Bluetooth. Unfortunately, even with those additions, Apple's iOS devices still aren't really the ideal way to play a game like this, as the onscreen controls are less than extremely precise, and putting your fingers on the virtual buttons tends to block out the action.
Not to mention that these games tend to be fairly hard, and using virtual controls makes them even harder. But if you're going for this one already, you're probably a Metal Slug fan, so you'll know the drill. Metal Slug X is available now for US$3.99 as a universal version. If you've never tried the series and want to see what it's like, I'd say wait for a sale, as SNK regularly drops the prices on these as well.