I just played this over the past week or so and got star 242 tonight after losing like 7 million lives.
Those Green Stars were pointless and didn't add any enjoyment to the game to me. It was just forcing me to watch cut scenes again and play through small parts of each level over and over and over. Ugh. Seems like a lazy way to draw out the length of the game. I loved just exploring the levels in Super Mario 64 and it would have been fun to have hidden stars or something there, but this Green Star crap is lame in Super Mario Galaxy 2.
While I'm at it, the level design was particularly lazy in the last few worlds (not all of the levels, but a lot). Its like the designers ran out of good ideas.
Furthermore, I'm f-ing sick of Star Bits. Having to bank 9,999 was lame. This goes hand in hand with the motion controls. While I don't mind the waggle for Spin Jumps and other things, the actions the waggling cause negate the need to use other moves. I think 90% of all tough to reach spots can be accomplished by Backflip + Spin. I think I used the Triple Jump all of once this game and that was to get one Green Star. I suppose the Triple Jump doesn't lend itself to these small 'planets' within each level. I wish we could go back to a regular controller instead of the Remote/Nunchuck combo. New Super Mario Bros. Wii's waggle was pretty well done. If I could just hold a regular controller (i.e. Classic Controller) and do that waggle for some things some of the time, I think that would be a good compromise.
This game is a very natural extension of the first Galaxy game (only somehow the camera was pretty bad in this game compared to the first). I think it was just a matter of "Hey, we have some more ideas and more things we can do with this Galaxy stuff that we didn't put in the first game (for whatever reason)." So they made the sequel, then filled it with some less-than-good level design in the latter half, then dragged it out even more with the Green Star crap. They should have just played the "Complete the game again with Luigi" card from the first SMG instead of doing the Green Stars.
I'd advise most people to just get the 120 Stars and call it a day.