The third Mario & Luigi game is fun, but it has a few downsides to it.
The plot of the game is that Fawful, the side-villain of M&L Superstar Saga (the first M&L game) has made a return to the Mushroom Kingdom with a sidekick, a brawny boar-like creature called Midbus, an army of loyal minions, and an arsenal of advanced technological weapons and gadgets. Fawful creates Blorb Mushrooms, mushrooms that cause toads to swell up to ten times their size, and therefore creates a disease called the Blorbs. This is just to distract Princess Peach, Mario and Luigi, and the rest of the princess's subjects, while Fawful focuses on the Koopa King. He tricks Bowser into eating a "Vacuum Shroom" (another of the green fellow's inventions) causing Bowser to go into a trance, inhaling everything he sees. He then returns to Peach's Castle, and inhales the Princess, Mario and Luigi, and a few more people.
At first the game sees you playing as Bowser, making your way up to Bowser Castle to win it back from Fawful. Halfway through the game Fawful's plot really unfolds. For the first half of the game you'll control Bowser (wandering through the Mushroom Kingdom) and Mario & Luigi (inside Bowser's body). Exploring inside Bowser's body is quite interesting; there are many areas, such as his Arm Muscle and Airway, and each area is explored like a mario platformer.
Sometimes when Bowser tries to do something, such as lifting a huge boulder, you need to go to a place in his body to assist him. If he was, say, lifting a statue, Mario and Luigi must travel to Bower's arm muscle to power it up by hammering energy balls into the muscle.
The game is roughly 15 to 20 hours long, considering you don't run from a lot of enemy battles and you search out every collectible. Don't worry; later in the game Mario and Luigi will be able to get out of Bowser's body and explore the mushroom kingdom.
Battles with enemies are different to most RPG games. In battles (be it either Mario and Luigi or Bowser) you face a number of enemies. Rather than attack you with just one attack that you can't avoid, each enemy has two or more attacks which you can avoid and counter - if you know how.
Enemies will usually telegraph their attacks before attacking. For example, a Goomba will lift its foot at either Mario or Luigi to show which brother it's attacking. You can then jump over it as it tries to tackle you, or jump onto it to damage it. Mario and Luigi counter enemy attacks with Jumps and hammer attacks, while Bowser crouches in his shell or punches attacks away. You also have special attacks to use. Sometimes in an area there are 10 attack pieces, and if you find them all you gain a new special attack. These apply to mario and Luigi, whereas Bowser gains a new attack when he rescues his captured minions.
However, in each new area you go to, there are usually only several new enemies to fight, and over time the combat just feels dull and repetitive. Also, on my playthrough, by the time I finished the game, I had felt that the game expected me to run from some battles. I defeated every enemy I came across and by the time I reached the games later bosses and final boss, I flew through with minimal effort.
Another thing I don't like is the story. Fawful is the games main villain, and he was also the sidekick of the antagonist in Superstar Saga. My question is, even though he was defeated in Superstar Saga, how did he manage to make his was over to the mushroom kingdom with a brawny new sidekick, many gadgets and weapons, and an army of minions? The game delivers no backstory on fawful whatsoever. It's more like, "Here's the villain from the first game popping out of the blue because we couldn't think of anything else." Usually when games have villains appearing seemingly from nowhere, it's a villain that hasn't been seen before. Not all games need a great story but for a Mario RPG you could certainly expect one that was more original; the story even reminds me of superstar saga.
The bottom line? M&L: Bowser's Inside Story is worth a shot, and a great game; if you're an RPG pro you'll want to give this one a miss as the game is relatively easy, and once you complete it you'll most likely have no interest in replaying it. Otherwise it's a nice addition to your DS collection.