Mars Volta. At first I thought they were thoughlessly harsh, but I've discovered that my kid's not such a knucklehead. He played it about ten times yesterday, and by the time I got done eavesdropping I discovered they're complex and fun. Geez, I was a real old fart on these guys.
Music you didn't like at first... but now?
Sometimes, there's a sense of discovery or a kind of magic that doesn't sink in right away. Have you heard something that you changed your mind about and wondered why you didn't get it sooner? What artists/recordings have you experienced this with?