I haven't actually played the game, but from what I've heard, the game is a nifty little doodad in which different types of plankton make different noises/simulates musical instruments. Eventually you can create a whole orchestra of planktons, and make your own tune. It's like a mini music synthesizer. And, well, that's it. The game is not any long lasting RPG or anything. Some people have lost interest in a short time.
Even though the game was originally only available online at Nintendo I think, I've seen the game in stores like EBGame for around $40CDN.