The Chronicles of Shannara 7/10
The source material is serviceable fantasy story. Honestly, the most interesting thing about it is the setting: it's set in a post-post apocalyptic Earth. The show has really high production values, which helps gloss over the mediocre source material.
I'm halfway through the first season, and I'm mostly disappointed.
The original novel only hinted at the fact that it was set in, as you say, a post-post-apocalyptic Earth. It was sword-and-sorcery foremost, and then if you read carefully you could piece together that the "Chaos Wars" were really a nuclear holocaust. This series hits you over the head with it from the get-go.
And while I understand MTV's reasoning for using the second novel as the basis for their show - the story of the kids of the people who saved the world appeals to their demo - it seems jarring to me. They picked up the story in the middle.
Finally, while we may disagree about the mediocrity of the source material, this show is less than mediocre. Oh, the production values are high, and the actors are all young and pretty, but overall it is pretty shallow.