I don't care for the new mechanics, how is the actual fantasy settings description? How big is the world described and how much details? Like, does the book provide enough material for you to set plenty of adventures in it, does it provide compelling motivations for quests, story seeds, hooks for relationships with NPCs or memberships in organizations?
I've always liked the "ancient greek" theme, but I also have a high standard for fantasy settings books (3ed Forgotten Realms and Rokugan campaign settings books spoiled me), and I am bit skeptic that the Theros book can compare. The SCAG for example felt miserable for my tastes.