I own all of Nightwish's albums and singles, and Century Child is the best of the bunch. As you can see from some of the other reviews here, Nightwish isn't for everyone. Angry teenagers looking for a thrash metal band should look elsewhere. Nightwish are as heavy as most metal bands, but they mix in a symphonic sound. I like to say they are loosely a cross between Iron Maiden (for the fantasy themes and similarities between the male vocalist and Maiden's vocalist) and Enya (for the fantasy themes and symphonic sound), but that's not entirely accurate; Nightwish have a sound all their own that cannot be easily compared to anything else.
Nightwish's other albums, including Wishmaster, Oceanborn, Angels Fall First, and the new Once, are heavier than Century Child. Century Child is somewhat a concept album, mixing in more symphony, creating a "beautiful" metal sound.
"Century Child" is by far the best CD in my collection, and Nightwish is the only metal band I really like. Metallica and Megadeth were cool when I was 14, but got old quickly. I always thought talented musicians could do more with the metal genre than has been done in the past, and I'm glad to find that there is a metal band which makes consistently great music. That band is Nightwish.
If you like metal, you may want to skip Century Child for now and pick up their other albums, saving the best for last. Century Child does have heavy songs (in fact, most of them are heavy) but the mixing takes away from that a little, while the others are no-holds-barred heavy. If you just like a beautiful sound and you're looking for something a little more than what most albums in your genre of choice have to offer, Century Child is a great album.
You won't regret listening to this album.