It's still a ridiculously fun game, but let's get a bit more into it.
The Mario and Luigi series has always been of extremely high quality. I can't think of one that I didn't enjoy from start to finish. That being said, there are some things that work really well and some things that don't work as well. But let's start with the story.
Peach, Mario, Luigi, Toadsworth, and a few other random Toads all get invited to the premiere of Pi'illo Island, a new resort built around the legends of the lost Pi'illo people.
Get it? "Dream team?" "Pi'illo?" Wait for it.
That's right, Luigi literally spends half the game asleep.
However, being as in tune with the dream world as he is, it allows him to alter and interact with the environment. He can summon up powers to speed up or slow down time, sneeze and blow gale-force winds across the stage, or summon a small army of himself that help Mario with drifting over pits, jumping to high places, or simply trampling enemies so you don't have to fight them.
In addition, Luigi can use this summoned army of himself to allow for some extremely powerful attacks, such as having Mario roll a ball of Luigis Katamari-style over other Luigis to increase the size and then knocking the Luigi-ball into the bad guys like a wrecking ball, having each Luigi boost the power of a fireball until it's huge and having Mario fling it at the bad guys, or having the Luigis balance on the end of Mario's hammer to form a giant mallet.
There's also moments where, when a giant boss is harassing the two, Luigi will instinctively tune in to the world around him (as well as his determination to help Mario) and turn into Kaijuigi, a giant unstoppable force that still manages to jump his own body height and wield a mallet as big as him, cube-square law be damned.
But here's my complaint. In all of those instances, it's still Mario directing Luigi and the other Luigis. It's not the army of Luigis on their own swarming the bad guys, it's when Mario uses a team attack to do it. Luigi doesn't even get to take part in normal fights by himself, he "merges" with Mario to add to his hit points, and if Mario does well enough on an attack a few Luigis will appear to do some additional damage, but again it's all dependent on Mario.
Even when Luigi is a giant, some of his most powerful attacks still rely on Mario being in control. Luigi's shoe is as tall as Mario at that point, there really shouldn't be much Mario can do besides feed him mushrooms to keep his health up or cheer him on from the background.
I would have loved to have seen a different take on this. Let Luigi be the boss in the dream world. Let him be the stronger brother and do the heavy work while Mario's completely out of his element in a world that's being shaped and influenced by his brother's subconscious (the fact that a scantily-clad Daisy never appears is a testament to the game's determination to keep it for all ages).
The controls are extremely tight and responsive, and while the 3D from the 3DS isn't necessary, it is pretty fun to use since some attacks involve the foreground and background of the fighting area.
Now, there are a few game play things I don't like.
For instance, I don't like the fact that if I lose a fight against an enemy (minor or boss), the game gives me two options: Try again, Try again in easy mode, give up.
I don't mind "try again" and "give up," but that middle option feels a bit patronizing to me. "Aw, poor widdle baby can't beat up a goomba? Do you want us to put it on the kiddie-level for you poor widdle baby?"
Forget you, game. Maybe I did screw up and engage the enemy in battle with only one hit point left, but by golly I will make it work.
I'd love to see more games starring the second banana plumber doing his thing to save the world, perhaps in a story concurrent to one of Mario's games where the older brother and Bowser aren't even aware of this huge, world-ending crisis going on that Luigi's keeping under control by himself.
But back to the main point. Mario & Luigi: Dream Team holds on to the high standards I've come to expect from the franchise and bring interesting characters along (minus Mario and Peach, of course, who are still as bland as ever). Between Bowser's underlings desperate to make their boss happy in order to get a promotion, a bad guy's secret fondness for wearing dresses, and the natives of the island (including two brothers who are essentially Hans and Franz from that one sketch taken to the next level), it's lots of fun to talk to everybody you meet and follow the conversations and get involved in their stories when they introduce a side quest.
If you do have a 3DS, I recommend picking it up. There are much worse uses for your time (looking at you, Yoshi's Island).