"Axis" is a lot less ambitious than its predecessor "Spin", but what it does it does well. It does not attempt to span decades of history, and so it avoids long stretches of tedium with which "Spin" is riddled. In "Axis" we don't have to watch the characters live their pedestrian lives while mysterious events unfold somewhere in the background. The main characters are always in action, and plot tension is sustained from beginning to end.
However, "Axis" is also a lot less conceptual than "Spin". It does not introduce new ideas, only elaborates (and only minimally) on the ones introduced in "Spin". My biggest disappointment with this book was that we don't really learn more about the Hypotheticals than we knew at the beginning. The idea that the Hypotheticals come to Earth to collect memories and upload them into some kind of vast cosmic network, is so basic and vague that it does not advance our understanding of the Hypotheticals very much. We still don't know why they want those memories, what they are going to do with them, or why did they have to move the Earth billions of years ahead in time. Or why did they attach a parallel universe to it, joined by the Arc. I wonder if the author chose an ignorant and simple-minded protagonist, a 12-year-old child, to communicate with the Hypotheticals, so as not to have to intelligently speculate about all that. I would have been more interested if the communicator was someone educated, especially someone who had studied the Hypotheticals, such as Lise's father. If Lise's father had enough material to write a book about them, he must have had more than just vague speculations?
Apparently the focus of this trilogy is not the Hypotheticals, but how the contact with them will affect humanity. At the end of "Axis" it becomes clear that the third book will be about humanity having to deal with uploading and the "5th age". This makes me look forward to it despite the underwhelming second book.
Although scientific mysteries go unanswered, "Axis" is still a good character-driven novel. With just a few brushstrokes Wilson sets up a wide range of believable characters. There is a determined truth-seeker (Lise); a person whose purpose in life is to preserve the existing order (Lise's ex-husband); a drifter who is nevertheless courageous and likeable (Turk); there are characters who and resist change kicking and screaming, like Lise's mother; and ambitious opportunists who seek to manipulate the change for their own purposes, like Dr. Dvali. For some reason secondary characters like Brian the ex-husband, and Dr. Dvali came through as the most vivid, while the protagonist, Lise, somehow didn't feel like a real person to me. But overall the diverse cast of characters and the plot tension made this book an enjoyable read.