AFI are like Napoleon Dynamite. Either you hate them to death or you love them to death. I happen to practice the latter because their music is so poetic and just, dare I say, beautiful. Give this CD a few listens to and it'll grow on you quick. Decemberunderground does not have the bonus track, On the Arrow (full of acoustic beauty), but you can get it on iTunes. It's well worth the price of the whole album.
This album is not filled with teenage angst. It's filled with emotion. If you research the background behind Decemberunderground, you'll find that it paints a picture of love, sorrow, and suicide.
AFI are excellent, positive role models: because of them, I am now vegetarian. Because of them, I promise to never drink or do drugs. Their music may sound "emo" or depressing, but they are probably THE most positive people you will ever meet. I gave Decemberunderground a 13+ rating because I feel that you have to be more mature to appreciate it fully.