Characters and scripts aren't quite as well written as superstar saga or browsers inside story. That's not to say it's bad, there are many creative moments and humerus characters throughout the game. With the exception of some issues I'll go into later on, the dialogue certainly doesn't let the series down.
It's also the longest of the Mario and Luigi rpgs. It took me almost 40 hours to complete. The turn base combat of the series follows very much in the footsteps of its predecessors with the new dream based combat supplying some minor variation. Though there's nothing really new, the series has some of the best turn based combat I've experience. The dynamic nature of these battles does mean the fights are slow and consume a significant amount of time. I'm not sure if it was due to the impressive length of the game or fatigue I'm starting to feeling for the series, but I found the combat started to drag towards the end. I'm hoping that they completely reinvent the combat for the next game, but I don't think this is a series that's prepared to break from the formula.
At its core it's is a good and entertaining game which I'd love to rate higher. However, it suffers from an intrusive tutorial system which frequently grabs control from the player (especially during the first 10 hours) to explain the blatantly obvious. For example, no doubt you've seen clips of this games dream world and creative ways dream Luigi can possess things in the background and then, though manipulate real Luigi's face, you can manipulate the contents of the dream. Perhaps you thought it would be fun poking at Luigi's face to see what you can do? No such luck. The moment you walk up to a new possess-able background, control is ripped from you and it is explained to you that you can possess the background item by pressing R. Then you're directed to the touch screen and told exactly what you have to do.
The tutorial permeates the game. I learnt to dread finding a new item because I'd be forced to scroll through a lengthy convocation needlessly explaining how and where to use it. This is frustrating and removes any sense of exploration or discovery.
Its not just tutorials with which you will find yourself needlessly interrupted. Your companions will stop the action periodicity to explain unnecessary things. For example, at some point you'll go to a part of Luigi's dream which clearly represents Luigi's paranoia and insecurity. It's beautifully made, with outlines of Luigi floating around, stressing about not being good enough. You don't need someone to interrupt the game at a random point just to spell this out. I'm giving odd examples here, but this problem is throughout the game, and had me screaming at my 3ds in frustration. It a real shame because at its core this is a very good game. Not on a par with the first or third in the series but a worthy addition all the same. Sadly the tutorial staff took what was ultimately a solid game and did their best to ruin it.
I'm not saying avoid this game. If you like the series you'll probably have a pretty good idea what to expect, but be patent and prepared to face mountain of tutorials.