You tap the screen of your iOS device to hover (holding your finger down to ascend), and you try not to die.
Machine Gun Jetpack is a spin-off of Halfbrick's free-runner, Monster Dash, which saw the debut of the machine gun jetpack, a device that propels Barry into the air and fires widly below him.
Like Monster Dash, MGJ is a free-runner, as Barry, strapped to the jetpack, is propelled unstoppably forward, gaining speed the longer the player manages to keep him alive. In turn, it becomes progressively more difficult to avoid the killer obstacles, which range from electrified clotheslines to guided rockets to lazers.
The obvious mechanic that sets MGJ apart from the many iOS titles that have followed Canabalt is that it's not about timing-based precision platforming, but timing-based hovering. But Halfbrick has implemented some other novelties into the game, including transformitive power-ups. In this build, I sampled the "bad ass hog" (also from Monster Dash), "lil stomper" mech, teleporter and gravity suit, which let me switch between running on the ground and the ceiling.
If you're quick, you can also gather specially-marked coins for the "second chance" slot machine, which pops up when you die (if you have a coin) and allows you to play for a chance to return to the game. Depending on how the slots match up, you can unlock other bonues -- or just turn up dynamite, which really ends your game (with bonus points).
Machine Gun Jetpack is still a few months out, but it was running smoothly on the iPad with a considerable amount of polish. I could gush about it more, but sometimes the most simple observation is best: I had fun and didn't want to put it down.