This review will contain spoilers!
In many sentences:
Keys shares many of the qualities that made the rest of the series great - it is full of imagination, humor, and interesting characters. On the whole, I enjoyed reading Keys and would recommend it to anyone who has just started the series, or is nearing its completion. Unfortunately, there were several elements that left me frustrated with this book.
The first frustration occurred when Seth returned the Sands of Sanctity to Graulus, healing him and kicking off the demon takeover of the Society of the Evening Star. Are we really supposed to believe that, yet again, Seth has done something stupid despite being directly ordered not to? And with something so precious? He really wouldn't go to Coulter first? Maybe the fact that I was so annoyed is a sign that I'm invested in the series and characters (which is a good thing) but I had a really hard time buying Seth's behavior.
What made this worse, however, was the next 30 pages, in which characters expound upon how people make mistakes, and it's important to not dwell on those mistakes, but to learn from them. I'm not saying these aren't important lessons, but where the rest of the book shows these things simply by telling the story, this time it is told to us. It's annoying, and while the book make be for children, it doesn't excuse stopping the story to teach us a lesson.
In fact, the remainder of the book is really good about teaching us how we need the help of friends, courage in the face of danger, and faith and commitment to what we believe is right, without ever mentioning them directly. Why couldn't the same thing happen with Seth? Why does Patton, and Bracken, and Coulter, all have to tell him this directly? Clearly the boy doesn't listen to advice anyways.
Greater than the annoyance above, however, is my annoyance over the ending. It was exciting, and interesting, but ultimately every little thing works out perfectly. No main, prominent character dies during the final battle, and they even bring several characters (The Fairy King, and Mendigo) back from the dead. The demon king is killed, and the rest of the demons are re-imprisoned. Kendra and Bracken fall in love. The Sorenson and Larsen families are reunited and they all move the Fablehaven. Every little thing works out perfectly. For a series that wasn't afraid to kill people off and be full of betrayal, this is ending is ridiculous. Why didn't they just plan to fight the demons all along if it was going to be this easy?
Despite my complaints above, I still had fun reading Keys, and I still recommend the series as a whole. Mull could have certainly improved the ending by making it a little more bittersweet, but the excitement, wonder, and adventure that permeated the series returns again in this final chapter, and for that alone it's worth a quick read.