I just started playing this (part 1) last week and am totally addicted to it.
So far, I am at around 89 stars, and haven't found anything that has caused me to want to throw my controller yet. The most annoying level so far has been the one where you race the big boo with the pull stars.
Last night I went to and beat the final Bowser level and did my first purple coin challenge on the Good Egg Galaxy.
Here's what I've completed so far:
Every single star galaxy with the exception of the hidden one outside of the garden galaxy where you have to feed the hungry luma; which I haven't unlocked yet.
Every 3-star galaxy plus comet and hidden star for the first four observatories, plus all these stars on the first two galaxies of the 5th observatory.
Two purple coin challenges: The gateway galaxy where you learn to fly with the red star, and the first comet in the first galaxy.
So what I have left are: The last star, the hidden star, and the comet star in the Toy Galaxy. All of the stars in the Molten, Dreadnought and Dark Galaxy. The hidden galaxy outside of the last observatory, And the 14 remaining purple coin challenges.
Like I said, I haven't been extremely frustrated yet, but from what I read here, that's coming up.
Two questions I have: Is playing as Luigi any different from Mario, or is it a purely I'm-looking-at-Luigi-instead-of-Mario thing? and also I read that collecting 9999 star bits turns the coconuts into watermelons. Does this affect gameplay at all or is it just an I'm-looking-at-watermelons-instead-of-coconuts thing? Seems like a waste of time if it's just cosmetic. There are hardly any coconuts in the game anyway.