Less than the sum of its parts.
I really wanted to love this game. The Sin City art style, the over-the-top violence, the chainsaw attached to hero Jack's arm. It came close, but it eventually felt samey, even boring.
The first few levels of MadWorld are brilliant. Not because they're especially better than the later ones (there are actually far more interesting levels later on), but because it's still new and interesting at that point. Slamming enemies onto spike walls, ramming signposts through their heads, and hurling them into chopping dumpsters is great...the first few times. Once you come to realize that every level has these lying around (albeit with added bits based on the current level's theme, like pinball bumpers in the casino level), you start losing the creative murder aspect that this game was built on. Once you throw a few baddies on a "rosebush," it loses that over-the-top feel. Sure, compared to other games, this is top-shelf violence, but once you start comparing the game to itself, it gets repetetive.
Luckily, there are enough redeeming factors to make the game stand out from other Wii shovelware. The commentators (John "Marcus Fenix" DiMaggio and Greg "Whose Line" Proops) really shine and the whole gameshow feel brings back fond memories of Smash TV. The Bloodbath Challenges are (for the most part) varied and fun, although they lost some of their shine when the devs ran out of ideas and just plopped in a Man Darts II. Multiplayer is fun, but make sure player 2 has played a couple rounds of the campaign. This isn't the kind of game where you can just rattle off the controls during a load screen. The original hip-hop soundtrack sounds like a bad idea, but fits perfectly, even if there are no real standout tracks. Finishing moves and Wiimote quick-time events are exceedingly satisfying. If MadWorld were presented as a series of violent WarioWare-style minigames, it might fare better. As for the story, this game would do fine as a "win the gameshow" kind of game. There's no need for the political tripe that the poorly-acted cutscenes present.
In the end, MadWorld is in the top 10% of Wii games, but it's best played in bits. Play a few levels, play something else, and it'll still feel fresh when you come back. Fresh and bloody. And chainsaw-y.