I thought the first half of the first season and the Yuki arc of the second season were terrific. The fairy part was kind of bland up until the last big battle (and a little creepy with his sister falling for him). GGO was ok, though it didn't really make sense for them to send a kid in for a criminal investigation. Action there was the far better part of that arc. And the remaining part did feel a little meandering until it got to Yuki.