Didn't feel like necrobumping the old thread. The Wikipedia article gives a good enough plot summary, it's as off the wall as ever and never takes itself too seriously.
Been playing this for a few hours now and it's amazing. I've heard it's better than the first in the series and I'm starting to agree. Bowser is tons of fun to play. His normal attacks are both charge types, which put me off at first, but they've tightened up the timing for action commands significantly. To compensate they've added 4 "tiers" of accuracy; OK, Good, Great and Excellent. Excellents are incredibly strict on some moves, and with charges they're a huge risk/reward. Don't expect to master the timing for any given attack after 2 minutes of practice.
The Mario Bros. play pretty much the same as they do in the first game, aside from the aforementioned timing changes. I haven't really gotten much playtime with them yet, so I can't say much about their new moves.
Honestly my only complaint so far is that the giant bowser fights require use of the mic, but it's nothing too extreme and the fights themselves are awesome, so whatever. Amazing game, highly recommended if you own a DS.
XI Wiki



Using 1.55b1 firmware + AR code to bypass the protection, apparently it can crash after saving sometimes (music continues but screen just hangs on black), but I haven't ran into it yet. Also released a new beta firmware apparently that fixes it without need a patch/AR code to bypass.
