In this week's New Yorker, David Denby sums up the film well:
Very little in "Basterds" is meant to be taken straight, but the movie isn't quite farce, either. It's lodged in an uneasy nowheresville between counterfactual pop wish fulfillment and trashy exploitation, between exuberant nonsense and cinema scholasticism.
and
But, in "Basterds," Tarantino is mucking about with a tragic moment of history. Chaplin and Lubitsch played with the Nazis, too, but they worked as farceurs, using comedy to warn of catastrophe; they didn't carve up Nazis using horror-film flourishes. Tarantino's hyper-violent narrative reveals merely that he still daydreams like a teen-ager.