“It’s dead obvious that creationism isn’t science, or even bad science. It’s nonsense. But I’ve long stated it’s also bad religion, because it doesn’t just take faith, it also takes a phenomenal disregard of reality.”
— Phil Plait, Bad Astronomy blog
Phil also recommends this video by a Christian who does a really good job of explaining why young-earth creationism and its various permutations should be unattractive for everyone, but in particular for Christians. I second his recommendation.