The bad thing about this game is you will have a seizure from all the colours if you play to long, but aside from that it's pretty fun. It's difficult to a degree but easy for the most part. you basically have to blow up colourfull enemies and get better weapons. Not bad for a starter wii game. ait uses the wii-mote enough for you to get the hang of it and play other harder games. Not much to it but fun enough.
Honestly, there is really not much about this game that is novel or innovative... in a action market mostly dominated by "3D" games. The real appeal of this game is its frenetic early 1980's style arcade action taken to an extreme. Anyone (like myself) who grew up thinking the Atari 2600 was the world's greatest invention is probably in the generation that would most appreciate this game.
Geometry Wars certainly lacks the depth of (good) modern platformers, first person shooters, etc.; but it more than makes up for this with pure adrenaline. Plus, the visuals are actually pretty impressive given intentional simplicity of the design.
There is also a bit of actual thinking that has to be done. The drone that assists you in play can be set to one of 8 modes and each mode requires considerable play to advance its skill level. Since skilled drones are nearly essential for acquiring "medals" on higher levels, picking the correct one and learning to play along side it effectively is an essential game element.
If nothing else, I think "Geometry Wars: Galaxies" is definitely worth the rental if you are looking for something that's fun and a bit different than most of the other Wii offerings. I'd especially recommend it if you are 35+ years old and want a bit of a flashback to the early 1980's arcade scene... without running out of quarters.