I just beat Bowser a few hours ago. Hell yes! It was a great and epic 'final' battle. The ending was cinematic and beautiful. Really, really great stuff from a great game.
Anyways, I have started collecting the post-game stars and it's a total blast ... for the exception of one level, which you can see in the video above. The entire level is designed to kill you. The green platforms fade away and don't come back and the yellow platforms start spinning and never stop. Oh, and the green slime? Instant death by drowning. Huzzah!
I'm still not complaining about it, though. I'm always up for a challenge. I just wasn't expecting such a difficulty spike and it caught me off guard ... about 16 times now.