Let me start off by saying that the average Wii gamer should play Super Paper Mario at least once. Whether they will like it or not is solely based on maturity and patience for somewhat long cut scenes, as the beginning takes at least 10 minutes to play through, and is somewhat boring.)
The blandness of the beginning, however, is more than made up for by the fun factor of it. It allows you, after Chapter 1-1, to play in 2D as Mario, but you can switch into 3D for a limited time to grab bonuses and get farther in the game, and there are many times where this is the required way to proceed in the game. Also, it has cool soundtracks for pretty much every villain. (Count Bleck, and Mimi, and... all of them. Really.) Last, though this may not be true for everybody, it has such a good plot that it has tons of replay value. (I beat it 7 times, to give you an idea.) By the way, fans of turn-based Mario combat (such as in Paper Mario and Paper Mario and the Thousand Year Door), take note: that has been done away with, as the game is a side-scroller and features the classic Mario hit-them-on-the-head combat.
The main theme is that there is a villain called Count Bleck, who wants to rip a hole in all worlds (there are more than one) called The Void. After The Void consumes all worlds, he will make perfect new ones. I won't give away the ending.
All in all, I would strongly disagree with the trolls on IGN who beat it to death. Super Paper Mario is a fun game in a fun series, which will hopefully last for a long time.