So at last (and a little later) I've finished The Raven Cycle series, and I can still say it is my favorite series. While I can see how some fans of the series saw it as a disappointing finale, I didn't feel exactly the same way. I think it was a good ending, but there were a few things I would have adjusted to make it feel more final. (Yes, I mean add more books.)
This review will be very spoiler heavy, so if you plan on reading the series, please read it beforehand!
Stiefvater's talent with characterization and interpersonal conflict continues to be strong in this book. The whole time, I was proud to see characters grow, confront each other, and come to resolutions. I was also excited to see a bit more of the supernatural elements with the demon, the undoing of Cabeswater, and Blue's father. Also, despite having Adam and Ronan's relationship spoiled, I was satisfied that it occurred closer to halfway in the book rather than for a brief moment in the end. It felt more natural and satisfying in that way.
There were also lots of new characters I enjoyed, mostly Henry, and also there were a few I was less sold on. I was never a huge fan of Piper, and her father and the rest of the triplets didn't really feel like menacing villains. While I was a huge fan of the demon being the main antagonist in this story, I had a hard time visualizing it and understanding it.
What makes this a good ending is that it closes up the main conflicts and allows things to come full circle. We get what we were promised: Gansey finds Glendower, Blue kisses Gansey, and Gansey dies. At the same time, our expectations are subverted because Glendower has been dead all along, Blue kisses Gansey because he's dying willingly, and Gansey is able to come back to life because of Cabeswater.
In a way, I can see why lots of fans saw the ending as a little bit of a cop-out; the things that are promised aren't what they seem. At the same time, the way this is done feels like that's the point. From the very beginning, even though I was aware of Gansey's death and Glendower, I always had this feeling that there would be something unexpected about these things. And I think Stiefvater did this on purpose. Not just to please fans, but because it's a much better ending. We are basically told that things are not what they seem, and these characters will still live on, even after the events over this particular year for them are over.
In the end, I'm sad to see the series end, but I feel like these characters are still living on, and that, to me, is a successful ending to a series.