The game is a blast to play, and being able to switch between the characters at will makes it even better. Jonathon is a melee character who uses whips, flails, axes, swords, etc. to take on the bad guys. Charlotte uses books to activate magic spells. When you've got them both together, you can use powerful "Dual Crush" attacks to smite your enemies where they stand.
The puzzles are overall relatively easy (compared to something along the lines of Professor Layton), but there have been some that I've needed to get online to figure out.
The only feature of this game that no longer works is the online connectivity, since the game is over 10 years old and not many people play it any more.
My one piece of advice? NEVER SELL ANY OF THE ITEMS YOU PICK UP IN A PAINTING. You will undoubtedly need them for a quest from the ghost, Wind, and you're unable to get duplicates of them. (This is something I didn't find out until after a lot of searching online.)