I read Illuminae back when it was released and really enjoyed. Then I bought Gemina when it was released, and held onto it in anticipation of the release of Obsidio, which came out last Tuesday. I read all three of them over the weekend.
You know how lots of reviewers will use phrases like "COMPULSIVELY READABLE," in all caps, like they are yelling at you? I would say that these books qualify, They are fast moving, exciting and heart-pounding. There is a lot going on, and things happen fast.
The format is very interesting, and is best experienced reading the physical book. Each page is an individual design triumph. The style is sort of "found footage," with analyst notes, chat logs, schematics, and other such elements. It's fun to experience, and gave me, as a reader, a sense of strong engagement.
One of my favorite "characters" in the books was AIDAN, who is the AI that shows up in all three books - always learning, always protecting, and ultimately experiencing love of a sort. The arc of AIDAN is fascinating.
There is a lot of YA romance in these books. Each installment has its primary couple, and those relationships, and the innuendo and banter that go along with them, frankly, grow a bit wearisome. This is a common failing, in my opinion, with YA fiction. I frequently find that the romance elements leave me cold, with or without the damned love triangle. Only Gemina has anything that approximates a love triangle, but even so, I found the cute sort of overwhelming.
While the format is a strength, it can also be a weakness. I did notice that the stories dragged a bit in the middle, before they really gear up for the conclusion, and some of the pages require a little bit too much work to read.
However, overall, I really liked this series, and I think that it has significant reread potential. The books are amazing physical objects and I am glad that I own them. They sustained my interest in the series through to a satisfying conclusion. I would definitely read more by Kristoff and Kaufmann.