I'm sharing my brother's copy, so I've only had time to put in 15 hours or so. Just beat Bowser, I'm at 72 I think. I'm really not looking forward to some of the obscene things I'll have to do to even get through the rest of World S.
Some people have called the game difficult. The term I would use for certain stars is a pain in the fucking ass. Now, tricky stuff like the purple coins on Flipswitch? I love that. But things that are made difficult by quirks of the control (after 15 tries I still couldnt beat the cocksucking tree slide) or that are just plain stupid (pretty much every destroy all the enemies in XX seconds level)? Not so fun.
Overall, there are some amazing levels and stars in the game, but I think I'd rank it a couple percent below Galaxy 1. That one had some of my favorite video game levels ever, this one wasn't quite so distinct in that regard, but almost made up for it with the weird crazy challenges.
