There's not a lot which hasn't already been said about Luigi, but here's some more opinions for good measure.
Luigi's Mansion. I'd played this one previously and knew what to expect. The training levels were omitted in the demo so there was the chance to have a look at these. I found that it is possible to suck up multiple ghosts at once from the training, something I hadn't managed before.
Quickly getting through the ghost ridden corridors which I had encountered before, I reach the first puzzle room. Here, there's a ghost in a rocking chair, you have to work out how to catch it. Every time you shine your torch on, he disappears. Waiting until he is yawning is the key to this one. You have to be very observant about the ghosts in this game, if they are hiding they'll react to what you are doing. Shake that cupboard over there, maybe that'll get the ghost out of hiding.
Once the ghost appears it's time to suck them up! The best technique is to pull in the opposite direction from the ghost using the control stick as if it was a fishing line. Of course, the ghosts are not going to keep still, you might end up spinning the control stick trying to keep up!
The first boss battle looks like one of Luigi's nightmares. Dark with swirling light all over the place. It's not too difficult to defeat the ghost, the puzzle is in getting to the boss battle.
None of this is too tricky, and puzzles are introduced gradually, with easier ghosts in-between. The more complex puzzles come up as the game progresses, these make it a lot more interesting, I'd like to have a play at the later levels to see what they've come up with.