At it's core it's a wave based shooter. In each hand you have a gun which you can toggle on the fly between 8 different firing modes. The first four are pretty generic, you have your single shot mode, your automatic fire, your high-spread shotgun fire mode and a slow-reloading grenade. But alongside these are a few more unusual configurations such as a laser pointer, which almost seem superficial until you figure out how to use them.
Each weapon configuration will have it's own reload/cooldown dynamic, so the automatic mode will overheat and slowdown if you just hold down the trigger.
Crucially, each of the 8 configurations is fun to use and feels great. Each mode has it's own strengths and weaknesses, but the audio, animations and general feeling of bad-ass is consistently great; which is handy as in those later levels you'll be constantly switching weapons.
The game is a score attack, where various robot drones come along and try to shoot you to death. Different drones have their own attack patterns, and there are also item boxes you and blast along the way to get temporary bonuses.
If you don't shoot down a drone before it fires back at you, you are left with two choices. Either try to shoot it's projectile before it hits you, or dodge it by moving your body. Tracking works great, and as enemies are confined to a 180 degree radius you can get away with just a couple of sensors if your playing on the Rift. Due to the aformentioned weapon cooldown mechanics, you are going to die pretty early on if you try to play sat down; as you'll frequently be in a position where you don't have enough shots charged to defend yourself otherwise. So It's very much a standing game.
There are some game mode variations, but none of these fundementally change the core gameplay mechanics, and the 'bullet-time' dodging reserved for the default mode is so much fun, I don't dip into the others much.
The interface does a great job of putting all the information you want infront of you post-game, including how you fared against your friends, the steam ladder, and your previous attempts. You can also filter the leaderboards down to specific weapons, so even if you're not that great at shooting (i.e. me) you can find yourself a nice little niche in the leaderboard to dominiate.