I bought this game back in late June, and after finishing my 100% runs through 64, Sunshine, and SMG1, I started it last Sunday. I just got the 120th star and started on the Green Star quest, leaving me with 123 stars as I stop for the night.
Overall, I love SMG2 so far. I can't say it's fully lived up to the hype, but it's been a fun game, and World S was absolutely amazing. The Green Star quest isn't quite what I expected to come after the first half of the game, but it certainly seems interesting (I say that now, watch me get frustrated and loathe it in a week, haha).
I must say, though, as logical as the concerns of this game being like SMG1 DLC seemed, I find myself shying away from even trying to call one better because they play so differently. They're not all that different, but for some reason I've found that the entire environment of SMG2 feels different -- and this is coming from someone who was just playing Galaxy 1 within a week before I started Galaxy 2. I can't say I like the soundtrack as much -- call me crazy, but I haven't enjoyed much of the music except for all of the nostalgic tracks (especially Buoy Base's music, my favorite music from Galaxy 1) and the final level's music -- but I'm thoroughly impressed by Galaxy 2 overall. Even though I've gotten as far as I have in little more than a week, the difficulty has definitely been upped from Galaxy 1.
I have to applaud Nintendo on a job well done, because Galaxy 2 not only inspired me to go back and re-experience and re-evaluate the other 3D Mario adventures, but it's been a great game in and of itself so far.