[I'm bored, let's write an unprofessional review. These are my opinions, feel free to state your own in the comments.]
Mario and Luigi Paper Jam is a simple crossover between the Mario and Luigi franchise, and the Paper Mario franchise. And I mean it when I say simple, as the game isn't the most impressive. Paper Jam is much more Mario & Luigi than Paper Mario, if that makes sense. The battle system, the dialogue, the art style, the music, the bosses. It's all 100% pure Mario & Luigi with a few new mechanics and of course, Paper Mario.
Battle System
To begin, this game has the most solid battle system of any Mario & Luigi yet.