Most addicting Live Arcade game that I've played. Try the demo. You'll probably find it worth the money.
It starts very easy (a little too easy, maybe), with few objects floating around the perfectly-sized map that you can shoot with ease. After about a minute, though, tougher AI appears, including the green blocks that follow you and you have to outsmart to destroy or the purple ones that break into more swirling pieces. The game gives many great enemy tactics to learn to work against and if you play long enough, you can learn the best ways to deal with each enemy type. And if you like this sort of thing (it's really a genre of its own) then there's a good chance you'll play long enough to do that. However, it gets insanely difficult and wildly chaotic after several minutes, and you'll need to learn how to use black holes, your weapon upgrades, smart bombs and the whole map nearly perfectly if you want to survive to get even 250,000 points, not to mention the other enormously insane achievements the game offers. You will probably learn how to dodge through dozens of enemies while blazing countless others down without thinking. It's a game that is constantly in motion, and surprisingly will keep you interested wave after countless wave of ingenious enemies.
A well-executed XBox Live mode (or any, for that matter) would've made the game a lot closer to perfect, but one can't get everything in a $5 Arcade game. Despite doing the same thing over and over, there is actually great replayability to it, and people who liked old arcade games like Asteroid or Galaga will probably have many hours of enjoyment from good ol' Geometry Wars.