I don't know why everyone hates this game; it's not THAT bad.
Yes, the story is incomprehensible unless you've seen the movie and/or read the novel that shares this game's title (Griffin's Story). I agree that the cel-shading is okay, but it seems like they just drew outlines around the blurry-textured models to hide their crappiness. Their mouths never move, which is dumb considering everyone's did in many older games.
In the PS2 and Xbox 360 versions, you can only "jump" to areas where there are floating orbs, and about half the time it doesn't work. That's what makes the Wii version a billion times better: here, you can jump to any spot on the ground where you can point the remote.
My only big complaint with the game (besides the graphics) is the fact that you can beat it in less than 2 hours if you're an even modestly skilled gamer. It's worth a rental, especially for fans of the movie, but you probably won't need to buy it afterwards.