This game is amazing. Wow.
Just beat Bowser tonight from start to finish in a ~4-5hr session with 3 friends, surprisingly enough. We took any shortcuts we found, the biggest one being when my friend in 1-3 pointed out, "Can we get up there?" I can't believe it skipped 4 worlds, funny enough, since before playing with them I got to (and stopped at) world 5 without skips half on my own (other half with my bros).
I'm a little sad the surprise of the final boss is over (that was too epic, seriously, EPIC), but I'm so glad I avoided spoilers for it. Hopefully this remains fun for my friends and I for a while, they haven't even touched like 2/3rds of the levels yet! Hell, we skipped 5 stages (I think) on World 8 alone.
Easily my favorite Mario yet. Nothing beats multiplayer goodness, NOTHING! We were just yelling at the top of our lungs half the time...
*Sigh*. It saddens me knowing this game is all we'll get (of its type) for a long time... After getting a taste of this, I NEED MORE.
edit: I can't believe I wasn't really hyped about this game until maybe 2-3 weeks before release (when more details were given). This takes game of the year. Easily.