The Reckoners series has been a mixed bag for me. Good characters and interesting ideas, but a tired genre (superheroes), and some muddle plotting. Sadly, this last book didn't redeem the trilogy.
I read the previous book, Firefight, not that far back, yet I had a little trouble finding my footing at the start of this book. Perhaps that's in part because I found Firefight itself muddled, but Sanderson could have done a much better job of providing a smooth transition. Once the book gets going, it levels out into Sanderson's usual skill in offering credible, engaging characters exploring an intriguing magic system. It's fun going along with the narrator, David, just because he's so likeable and earnest.
In fact, the characters are so likeable that it feels like a series that could go on for quite a long time before running out of steam. While I was reading, in fact, I was thinking, 'The next book should be called Calamity, where we finally learn who Calamity is, and David faces them.' - completely forgetting, of course, that this book is called Calamity. That made the ending all the more of a letdown. Because we do meet Calamity, David does face them, and ... Sanderson decides to drop his pen and go home.
I can't overstate how much of a muddle this ending is. First, [spoiler] David finally develops powers that make absolutely no sense within the Reckoners scheme. Second, the ending itself makes little sense. The nature of Calamity is only vaguely explained, and then pretty much dropped. And then everything is happy ever after, which was the only part that fit the series. The whole nature of the superpowers aspect - the technical core of the series - is pretty much tossed aside.
To me, it feels very much like a longer series was planned, but then, for reasons of time or focus, Sanderson decided to end the series early, and just tacked on an ending in the last fifty pages of a mid-series book. It's so disappointing that I can't recommend anyone start the series at all.