Well...its not exactly co-op in that sense. One person can control a second floaty thing to freeze enimies, collect power ups, but they don't exactly play with you. Still, its a nice feature.
The game is amazing. The art, the music.
Even if it seems pretty simple and easy at times, sometimes thats the best way to make you complete a game, and even when you complete it, theres still the fact you have 100's of stars more to collect to 100% complete the game, which on this title, you probably would end up wanting to do by completing all the galaxies again with the different tasks.
I'll tell the truth. I hated Super Mario Sunshine, and I never played all the way through Super Mario 64, but this has something.