As usual the gameplay and combat is really varied and every new enemy encounter and boss feels fresh and inspired, especially with Paper Mario along for the ride. I like the Bros. Moves they put into this (although a few are recycled) and like a lot of sites have said, they keep tutorials to a minimum in this one.
That being said, I'd say the weakest point of this game is its story. There's essentially no huge twists; it's your average Bowser-esque storyline. The other M&L games usually have some other antagonist entirely (if not a secondary antagonist to Bowser) and this one just seems really straightforward. It's not awful, per se, but besides the whole "Paper" aspect that's going on, it's not the most unique story either. When it comes to the humor...I'd say it's alright. Some say they find it weaker in this game than in previous entries, and maybe so. It might be a bit lacking in self-referential humor, but I still think there's enough enjoyable lines in the game to be worth it. Especially from Starlow.
Oh yeah, and there's also the Toad Quests that people don't like because you have to do like 30-40 of them and some of them are puzzles and others are hide and seek. Personally I didn't find it too aggravating (at least until towards the end when I did all the optional ones for 100%. There's 70 quests in all), but that's something to keep in mind if you really dislike tedious minigames.
But overall, I'd probably give it like a 7.5/10. If you enjoyed all the other M&L games, I'd still say you'll probably enjoy this one...although maybe not as much. But that just depends on whether you place more importance on the gameplay or story, and how much you dislike the Toad Quests.