I dunno how many hours of gameplay there is but there are 120 stars, and you need about 70 or so to finish the game.
also, I won't say here but the game has huge replay-value, i.e. there is a nice unlockable when you finish the game with all stars collected.
From what I've heard from the IGN wii podcast, (matt's 60stars in), it's going to be freakin awesome. While reviews don't matter for this game, matt hasn't ruled out the possibility of giving it a perfect 10 though admittedly he doesn't know what score he and bozon are going to give it. Well worth listening to imo, they have NOTHING BUT PRAISE for the game.