On its own, it's a quirky, unique, and very fun RPG, but most of all, it's a sincere love letter to the Mario universe.
The game alternates between playing as the Mario Bros. and playing as Bowser. For most of the game, the Bros. are inside Bowser's body (hence the title) running and jumping through 2-D platforming stages reminiscent of the classic Super Mario Bros. games. In contrast, Bowser spends the whole game traversing through open 3-D worlds in the Mushroom Kingdom, punching and burning up anything that gets in his way. The two different styles actually complement each other nicely, especially with all the ways the Bros. and Bowser work together. For example, In one section, Bowser uses a "Boo-Ray Machine"which changes the behaviour of platforms so that the Bros. can progress. The Bros. help out too in a wide variety of minigames, from rhythm-based hammer challenges to increase Bowser's arm strength to shooting balls to stimulate a mortally wounded Bowser with a shot of adrenaline. There's plenty of variety in the individual stages too, especially with all the new techniques the Bros. and Bowser get to tear down environmental puzzles. Not everything works well (the touch screen based pollen mini-game is especially frustrating), but with so many styles, you won't get bored.
Of course, that is all mustard on the delicious bread that is the battle system. Bowser's Inside Story adds Bros. Attacks to the Mario Bros.' arsenal, which start off as relatively predictable (the Bros. kick a Koopa Shell back and forth) but become quite outlandish (Luigi gets stuffed with food and does a Ground Pound with his massive weight). These attacks have button combinations which are simple to learn, tricky enough that you have to pay attention to them, but not so difficult as to become frustrating. Since you will rely so much on them, you will instinctively work to keep both Bros. alive, since their individual attacks are much less effective. There are game balance issues with the attacks though, namely some are far less efficient than the ones before it. I managed to get through much of the game relying on Jump Helmet, the third of 10 attacks.
Also new to the mix are badges, a very rewarding mechanic which is based on how skilfully attacks are pulled off. On top of the simple, yet sophisticated and involving mechanics as well as the wide variety of bosses requiring unique strategies to defeat, there is a Challenge Node which hones the player's aptitude at the battle system even further with upgraded bosses and skill challenges based on the Bros. Attacks. While the game itself is not too difficult, the "X" bosses in will seriously test your strategy and reflexes, especially the last of the lot. I won't spoil who it is, but it's one of the most satisfying bosses I've ever faced in gaming, pushing all your skills to the limit.
Bowser doesn't have the insane gymnastics the Bros. do, but he makes up for it in sheer force. While not as fleshed out as the Bros. battle system, he has a bunch of tricks of his own which use the Touch Screen to control his minions. Whether it's setting Goombas on fire or Shy Guys using Bowser as a slingshot, it's all very fun. In addition, Bowser gets his own attack-based challenges involving using his minions to, er, give a Block lady a massage. Yes, it's a crazy set-up. Again, there are balance issues, as the later attacks get easier to use despite being more powerful, with the ultimate one requiring almost no skill. His hardest to use attack, Bob-Omb Blitz, is also his worst.
Again, the game implements a great teamwork mechanic in which Bowser can suck up small foes for the Bros. to take on. This is used cleverly in the final boss fight in which Bowser taking out the first phase and using his vacuum skill leads to a fully fleshed-out second phase. Bowser has one more style of gameplay: the fully touch-screen and microphone controlled Giant Bowser battles. These are pretty easy overall, but they are some of the best uses of the DS's capabilities and they make you feel like a total badass.
On top of the fantastic gameplay, we can't forget the entertaining and even surprisingly touching story. While the Mario games have traditionally emphasized gameplay above all else, this game just wouldn't be complete without the engaging narrative. It reads like fanfiction, and I mean that in the most complimentary way possible. While the game positions the much-loved Engrish-speaking Fawful as the main villain and introduces a whole bunch of quirky characters, at its heart, it's all about the familiar Mario universe characters we know and love. The fact it's set in the Mushroom Kingdom and focusses on the plight of the Toads makes it feel more relevant as a Mario game than the other Mario & Luigi games. It even goes some way towards making some of the characters more likeable than they've ever been in the main series. Peach, for example, is actually shown doing some decision-making for a change, a marked improvement over her usual "brainless ditz" portrayal in recent games.
Bowser, of course, is the star of the show, and while the game makes it clear that he's still a villain and the same lovable idiot we've become accustomed to, you'll actually gain some respect for him over the course of the game. His unrelenting determination is put on such display that even the gameplay feels it. As well, you feel for his minions and the sheer admiration they hold towards their Koopa King. The game goes beyond the typical video game characterization of monsters as simple brutes that only exist to be killed and gives them a human quality. You come to understand why they would serve a bad-tempered and dim-witted brute like Bowser, as even when he's yelling at them and calling them idiots, he still cares for them in his rough way. The part where Bowser rescues the Koopas and they chant his name while Bowser gets an "Aw, shucks" reaction always gives me warm fuzzies inside.
And of course, there's the ending. It's a simple gesture, yet to any devoted fan of the Mario series, it means everything. It's a culmination not only of the game's events, but of the symbolism and characterization that has been established over the entire series, and it's one of the most touching scenes in gaming. At first, I was dismissive of how supposedly heartwarming the ending was, but then I saw it, DS in hand, and wow. Just wow.
Overall, Bowser's Inside Story is a very well put together game I would recommend to anyone, but to fans of the Mario series as a whole, it achieves on a much greater level. Third time's the charm for the Mario & Luigi series, and Bowser's Inside Story ranks as one of the very best Mario games ever made.