Easily overlooked, maybe even dismissed as a port of an Xbox Live Arcade game, Geometry Wars is worth every single penny.
What you have is a fast and furious shoot 'em up akin to Asteroids meets Galaxian. The variety of enemies to shoots, level design, steep learning curve with consistant unlockables to improve your ability means that you will lose hours to this game.
Basic concept is that you are in a wall 2D part of space with enemies constantly spawning and attacking you. You need to shoot them down and avoid collisions to survive. When you shoot an enemy they drop a geom which is both currency to unlock sections of the game and a multiplier for your score. You will need to collect up to 150 of them to help boost your score to the dizzying heights needed to gain a Bronze, Silver or Gold Medal and progress through the game. However, if you die, you lose your Multiplier and go back to 1x and have to build up again.
The wiimote + Nunchuck interface lends itself well to the game and feels very natural so do not fear that a lack of dual analogue sticks is going to affect gameplay.
2 player coop mode is a fast and furious team effort, sharing lives and smart bombs (which you will need when the going gets tough) whilst the competitive games have you fighting to collect geoms and get the higher score before the level becomes too tough to survive.
I'm not sure I can praise this game highly enough. In a sea of games relying on top notch graphics and Hi res Plasma screens it is nice to know that there are still games developers out there who want to make their games playable and FUN, not to mention a challenge with a long lifespan.
BUY THIS GAME IF YOU HAVE A WII.
(Additional: The DS version is the exact same game. Having both versions will unlock certain new levels and allow you to sync them together over WiFi seemlessly, but most people will not need to own both copies)