I had fun with SMG2. I don't understand why people consider it to be as great as the first game, or why other people even have the balls to say it's better than the first game.
Half the level design is just been there, done that and the music is basically what you've already heard from the first game. The only standouts are Melty Molten Galaxy, Bowser's Galaxy Generator and the Final Bowser Battle.
The hub world is nowhere near as imaginative as Rosalina's Observatory as it's just...a ship the shape of Mario's head. And Luba himself...how do you go from an awesome princess with an emotional childhood backstory to a space version of Patrick Star?!
As for the story itself...should have been the plot to a New Super Mario Bros game. In the first game, Bowser was raining destruction all over the Mushroom Kingdom. Here, he turns big (No explanation, no build up, you just see him bigger) and captures Peach so she could make him a...cake. I know I shouldn't complain, because it seems like a Bowser thing, but that shouldn't be a story for a sequel to an epic Mario game!
Overall, I feel that Galaxy 2 is just a glorified expansion.