I thought the book was really well done and I probably would have loved it while I was the appropriate age group. As it was, I didn't personally love it a ton--I think I hate reading stories where the main character (or one of them) causes all the problems in the book and gets him or herself into scrapes that you just know are going to be big problems for them later. I just hate that and get more and more uncomfortable as I read, and try to avoid picking a book up when I know that's going to happen. This book relied on Seth breaking every one of the grandpa's rules and getting everyone into more and more trouble, even though he knew the consequences of breaking them--and that really got annoying after a while. I also felt like Mull avoided several really important plot points by having the grandparents say, "We don't have time to talk about that now. We'll have to talk about that later." Like the grandma wouldn't explain why she was a chicken? Why couldn't she just tell them in one paragraph? I'm guessing it will come up in a future book, but I felt like that was a cop-out.
Overall, I think the book was great for middle-grade readers. I bet the next ones in the series will be even better, but I'm not planning on reading them myself. I will heartily recommend this to Dane, though.