Adventure Time's pilot aired in 2008, and since has collected a cult following on the internet. A boy and a shape-shifting dog save a bubblegum princess from an ice king...such a setting is delightfully fanciful, even for fantasy! For those who loved the original, there may be some differences that make you uncomfortable. For example, several of the voice actors are different, and the character designs are slightly altered. However, as long as you came for Pendleton Ward's brand of humor, you'll be more than satisfied.
Not everyone is going to like Adventure Time, however. There are many who simply have not been exposed to such an exotic type of humor, and so this show may frighten rather than entertain.
I'm sure, however, that the consensus is that Cartoon Network has improved it's show/success ratio by green-lighting Adventure Time with Finn and Jake.