I think I'm officially the slowest player here...Of course when I bought it I was told by my wife that it and Zack & Wiki were to by my Xmas gifts so I didn't get to start playing until after Xmas. Then my time (limited as it is) was split between Z&W and SMG... still have to beat Metroid, but I'm only about 2 hours away from that one. Given that I only have about 1.5-2 hours to play at night at the sacrifice of sleep, SMG and Z&W are good games for then, other than the fact that both are extremely addicting so the short levels, while allowing for short playtime, turn into many levels played.
I'm at 95 stars, still haven't beat the game, I'm just collecting stars for now. I've only done the Gateway purple coins, but I've only got about half of the Dreadnaught and all of Melty Molten left.
If I never have to race the other Mario again I'll be happy... Swimming was a pain but it took until this thread to realize you can hold A... so I was ~90 stars in before I realized that...Now swimming is much easier.
Spring Mario is the spawn of Satan. I hate any mode where letting go of the control stick leaves you moving forever... if you eventually slow down and stop it's fine but you can't just stop to look around as spring Mario.
I haven't really had too much trouble with much else, provided I take my time. Daredevil Galaxies are tough but they usually leave you with a method of getting a 1-up or two so you can play them indefinitely. I have more difficulty with the Cosmic Comets.