Since plenty of reviews have preached this game's virtues since it's release, I want to focus on one thing in particular which has been underplayed in my opinion. The two player mode in Galaxy 2 is great, in the right circumstances. For two gamers, I can see how it isn't that much fun. But for a gamer and non-gamer, or gamer and young child, the two player mode is a blast and adds so much to the game. My girlfriend and I play the game together. I bought it as a single player game, planning on playing it alone. I bought New Super Mario Bros Wii around the same time so that we'd have something to play together. We've played some of that, but since she's a casual gamer, she gets frustrated by dying in the game.
We tried the two player mode in Galaxy 2 out of curiosity. During the first level, I didn't really think she'd like it, there didn't seem to be that much to do and it didn't seem to be all that helpful to have a second player. But she did like it so we continued playing and the more we did, the more fun she had with it. And not only that, but having her playing really is helpful. I'm a decent but not great gamer and Galaxy 2 is definitely more difficult than the first Galaxy game. There were levels I definitely would have struggled with had she not been playing. She enjoys helping and doesn't have to worry about getting frustrated by dying.
The second player can do a few things.
1. Pointing at the screen and collecting star bits, in the same manner as player one would. This is helpful because it means I can focus more on the platforming and not as much the collection aspect, which again is helpful on harder levels.
2. Pointing at the screen and pressing A to collect coins, 1 up mushrooms, etc. This can be really useful to collect things in areas that are not easy to get to. Also very useful on stages with coins that require speed collecting, which we find very fun.
3. Pointing at enemies and pressing A to freeze or kill enemies. She can either kill smaller enemies or freeze bigger ones in place, like chain chomps or firebars. Very useful on difficult levels, I'd have trouble avoiding all of the enemies without her help.
Again, the two player mode isn't meant for two gamers but it's a great fit for one gamer and one non/semi-gamer. The game as a whole is a blast and all Wii owners should play it.