it's a little anti-climatic. You see Rosalina at the end, which is a nice addition (and there's a 1-up behind her house, just one last taunt at you), and she joins you on her ship, but just says "Thank you." and nothing else. You don't even get to see her "finish" the story to the Lumas in the library like you did the first 120. Almost everyone else on the ship but the big purple Luma just keeps giving you advice or telling you to get Green Stars, Power Stars, and Comet Coins, when there's none left. Even the red one, which I expected to pull a Mario Galaxy and let you have flight, even if it was just for Starship Mario. Then I got to the message board to see my victory image, and it was a picture of the star icons. The original Galaxy Luigi photo is still the thumbnail for my Photo Channel, so I was pretty disappointed not to be able to replace it with anything worthwhile. Does Mario get a different photo like in Galaxy 1?
The gameplay is its own reward, of course, don't think I'm not saying it was already enjoyable to have this extension on the game. But it would have been nice for at least the people on the hub to know you did it, instead of telling you to keep going for something that's not there.
By the way, is there anything special about the Engine Room, other than it just collecting the power-ups you got? There's a glowing circle in the middle I didn't notice before that's suspicious, but it doesn't react to Mario or Luigi. On such a crowded ship, it feels pretty useless.