While some aspects of this game are nostalgic and fun, there are some that are just frustrating.
The story starts with the cliche of the Mario series, Peach getting kidnapped (You'd think she'd be used to it by now). Mario, of course, blames Bowser, and he and Luigi go to his castle to get her back. After a funny bumble by one of Bowser's minions, it turns out that Peach isn't there, and a mysterious figure by the name of Count Bleck arrives and transports Bowser and Luigi to a wedding chapel where Peach happens to be, and Mario to a strange town called Flipside. At the chapel, Bowser and Peach are forced to marry. This bond of hatred creates what is known as the Chaos Heart, the first step in Count Bleck's plan to fulfill the prophecies written in the Dark Prognosticus. Mario is then met by a recurring Paper Mario character, Merlon, who tells him that if he does not gather the eight Pure Hearts, all worlds will be drawn into the void created by the aforementioned Chaos Heart.
The gameplay is indeed very nostalgic. It plays like a combination 2-D platformer RPG game. It feels like what Paper Mario was meant to be - 2-D. You jump on enemies to damage them, or you can use the ability of a pixl. I, personally, think the pixls are bland and outlive their use quickly. Anyway, you can also point to the screen to gat information on stuff, or reveal hidden things. As you proceed through the game, you'll meet back up with Peach, Luigi and Bowser, all of whom you are able to switch to at anytime, each with their own unique abilities. Peach has her parasol, which can be used as a shield or a parachute. No problems there. Bowser has his flame breath and brute strength to rely on, he's very useful in combat. Luigi can jump. High. Mario has (ugh) the Dimensional Flip. It allows him to flip from 2-D to 3-D for a limited time. It's very awkward. The puzzles for it are either too easy or too hard, the camera means you can be ambushed easily if you travel backwards, and there's almost always something to access in 3-D. So Mario will almost always be the one you use, because no one likes to miss out on stuff.
All in all, it's a good game, and the ending, while similar to Thousand-Year Door, simply blew me away. I'm not going to recommend it, but I'm also not going to discourage you. Do what you will, I say the storyline is worth it.