As for me, I'm at World 4 with 53 stars collected. That's in just three sittings. You might say the game hooked me.
Seriously, Super Mario Galaxy 2 is rocking my socks off. The level designs are so good, so bloody good, I'm struggling to think of which ones to single out. For the sake of conversation, every level with the Bulb Berry power-up for Yoshi has put a huge grin on my face. Each of the Luma galaxies houses an amazing one-time concept - Flip-Swap, Beat Box - made all the more memorable because these ideas appear just this once. The Chompworks Galaxy has also stuck in my head; I love how you alter the environment to keep the Chomps rolling to where you need them. Oh, and the slide in Tall Trunk is just too cool for words.
I've really liked the bosses so far. Yeah, there are a couple of reprises from the previous game, but everything else has been very impressive. Rolladillo, Megahammer, Glamdozer... it's cool how the mechanics introduced in the level are used in the boss fight, especially Glamdozer's fight.
The music has hit it out of the park for me as well. I was concerned SMG2's soundtrack wouldn't have the same impact on me as the first game. This concern was squashed as soon as I heard Sky Station Galaxy's BGM. Hats off to the composers again, and that includes some of the non-orchestrated songs too. Puzzle Plank Galaxy has this incredible call-and-response with a fiddle piece that sounds like the sort of thing people would square dance to and unusual synths. I never would have thought that musical combo could work -- I was wrong! I must also mention the Haunty Halls Galaxy BGM, which is a wonderful remix of the Ghost House theme from Super Mario World.
To all the cynics out there: I really think you should give this try. You might argue that all this stuff could have been in the original game and that this is "basically a level pack". Well, while that may be true, there's no reason that should bother you whatsoever. This is a purely creative and enjoyable endeavour that reminds me why I still play videogames. Come on, why are you still reading this? Go buy Mario Galaxy 2 now.