One of the Great RPGs
Let me start by saying that the original Paper Mario was what got me seriously into games. I had played some pokemon before that, but I had never been a serious gamer. Paper Mario as a series has captivated me with it's unique art style, humorous writing, and powerful gameplay. The Thousand Year Door is no different.
The story starts as Mario gets a letter from Princess Peach, who has found a treasure map and wants him to meet her at a distant town called RoguePort. Well, the Princess gets captured by some evil baddies, and Mario has to find seven crystal stars in order to save the world from an evil demon. The story isn't great, but it moves the game along and is bearable thanks to the series' great characters.
One thing that Mario RPGs have done well is combat. Rather than having random encounters, the enemies appear on the overworld and it's up to you to either chase them down or avoid them. Once you are in combat, the game requires you to time out your hits in order to do the most damage. Mario can jump on or hammer enemies, but he also has his partners, an array of colorful characters of all backgrounds. These support characters have their own strange abilities and can make these battles frantic.
What has always separated Paper Mario from the crowd has been the graphics. All the caharacters are in 2-d(Thus the Paper part) in a 3-d world. Paper Mario:TTYD expands on the paper element by allowing Mario to transform into a paper airplane, boat, or roll up into a tube. The soundtrack is solid, bringing the player deeper into the experience. The music travels perfectly with the emotion of the game, from spooky to intense to heartwarming.
As for replayibility, I still pop this game in occasionally. It's a lot of fun that can suck you in. And with the fall of the gamecube, you could probably get a used version of the game to play on your Wii for $20. I suggest you buy it. This is, in my opinion, a true masterpiece.