Batman is the role West was born to play. He delivers his lines with a seriousness and self-importance perhaps matched only by Steven Seagal--and Seagal isn't trying to be funny.
I can understand how comic-book fans might dislike this movie. It does, after all, treat the whole Batman concept with jokey disrespect (though really, as another reviewer pointed out, it's an over-the-top parody of the old serials). However, for those of us who see the inherent silliness in the notion of a "millionaire playboy" dressing up in a bat suit to fight "supervillains," it's fun to watch a movie that sees it as well.
Perhaps the most amusing aspect of this movie is its off-the-wall view of the United Nations; the particular ambassadors are treated as something more than bureaucrats, apparatchiks, and political cronies who could be replaced in five minutes with any of ten thousand equally capable (or incapable) people.