What follows is my history of interaction with the Kingdom Hearts franchise prior to playing the Dream Drop Distance demo:
I've never played any of the games, not even at a friend's house, and I've only seen a few Youtube videos that didn't endear it to me much. Most of that problem came from the fact that the Disney characters just looked wrong rendered in 3D. I've also made a couple of vitriolic statements about it in other threads, but please don't go looking for them - suffice to say that it was a pretty major case of Aesop's sour grapes. I don't own a PS2 and I've heard that the spinoffs play pretty differently to the main series.
Anyway, when I heard that Dream Drop Distance wasn't going to mess with the gameplay (much), by that point I'd already been a 3DS early adopter and was somewhat interested in the game. Finding out it contained characters from the best DS RPG ever, The World Ends With You, only made me want it more. Then the demo got released, and after a few technical issues with the store, I downloaded it.
Let's get the elephant out of the room: no, you don't get the actual side characters as party members, just those colourful minion things. But I don't think that's too much of a bad thing - they can still fight, and besides, those side characters probably have better things to do than follow Sora everywhere anyway. The Flowmotion system is surprisingly fun to use, and while switching to the D-pad to pick out abilities in the middle of a heated battle gets annoying sometimes, the combat is otherwise pretty fun.
Graphics look nice - yes, Ursula does appear in 3D for the tutorial, and it still just looks wrong but I can manage. Everything else is all very bright and colourful and it's easy to make everything out. Also, I don't use the 3D on a lot of stuff, but I tried using it for this and it actually works surprisingly well. The music (or what I heard of it) was decent as well.
Not much story gets brought up in the demo - there's a cutscene where Sora meets Neku, you fight a few enemy groups and a boss, then it ends. Mostly it's for series veterans to see if they like the new mechanics - but speaking as a non-veteran, I enjoyed it.
Won't pick up the game on launch day, but I will probably get it at some point unless it gets slammed across the review board - from gamers as well as journalists.