The best game ever! GET. IT. NOW!!!
The Good:
An enormous number of galaxies, all unique and incredible. While it seems like a bit of a job to collect power stars at first, you will never get bored of it. Every galaxy is so incredible, it could become its own game. I was never disappointed once by a new galaxy showing up. This game is perfect.
New power-ups change the gameplay, but I'd have to say the most notable thing about this game is not Yoshi, but actually all the amazing new galaxies. They all use different elements, and all the boss fights are epic.
This game is addictive. Beautiful graphics, and a beautiful soundtrack. Everything about this game is great.
The Bad:
The only bad things about this game is the lack of a story, but that's not too big of a deal. The gameplay itself serves as the story.
Another problem is the lack of involvement of P2 in Co-Star Mode. I wish someone could jump in as Luigi and play by your side, like in New Super Mario Bros. Wii. Oh well, I guess that's too much for Nintendo to handle.
Conclusion:
Anyway, this is the best game ever. If someone threatened to either throw away Super Mario Galaxy 2, or all of my other games, I would choose to keep SMG 2 and leave my other games behind. This game is THAT GOOD.
So, why are you still reading this review? Go get the game. NOW.
You won't be disappointed.