I just finished playing the demo.
I have a weird relationship with M&L games because I think they're really awesome, but at the same time, I don't like them. (I like Paper Mario, though.) However, I really liked the demo. I really enjoyed the Dream World fights, fighting with the power of multiple Luigis on my side. I was very surprised when I realized that the characters weren't actually 3D. All this time, I thought they were 3D models...
I really don't like the standard M&L long, long boss fights. It's one of the main things that turns me away from the series. In the demo, I actually died against the Dream World boss after fighting for quite a while.
Of the M&L games, I did like Superstar Saga the most, and this game reminds me of it. Of the gimmicks they added since Superstar Saga (Baby M/L, Bowser and his stomach), I like the Dream World gimmick the most by far, so I definitely want to give the full game a shot one day.