This game is ok, and I don't regret renting it. However, I returned it before finishing the game -- because I grew bored. The earlier "Mario and Luigi" games were far better, in my opinion. The novelty of being "inside" Bowser was certainly fun at first, but I think this game is lacking in some of the key role-playing/leveling aspects that make a game like this addictive.
In other words, I hate it when every other fight gives you new abilities and cut scenes, etc. It feels like most of your time is spent learning new abilities and not enough time is spent USING them to fight and solve puzzles.
Also: the DS is getting a bit dated, and I'm playing more and more PS3 and XBox games. So, playing this latest Mario and Luigi game felt like a pretty big step backward (whereas earlier editions had more novelty and newness going for them.)
Anyway, if the DS is your main platform and you are looking for a decent game, this one is worth a rental. I'd play the earlier titles in the series first if you are new to these games. Superstar Adventure was excellent, and better than Bowser's Inside Story.