I have the urge to start comparing this book to other books-- two other specific books, to be exact. I want to compare it to Oh Pure and Radiant Heart by Lydia Millet and Geek Love by Katherine Dunn. The comparisons are, in their own way, cheap shots. Both books have an obvious point of overlap in the subject matter (the interest in the bomb testing grounds in the Millet and the idea of reclassifying normal personhood in the Dunn). All three books seem to me to do the same thing (to greater or lesser degrees) where they paint themselves into a corner with their good ideas.
As you can guess from this description, I found the second part of Half Life somewhat disappointing. While I enjoyed the obvious energy and skill of the writer, the prose pyrotechnics (lists, poetry, dream sequences, etc.) started to grate at a certain moment. In the first half of the book, I found the various non-standard narrative elements exciting. By the second half of the book it felt busy, and a distraction from the too-obvious fact that the plot was not holding up very well. I still enjoyed reading the book, but I just cared less and less about the fate of Blanche and Nora. At least a little bit less would have been more for this reader.
(If you don't know, Jackson is writing about a hypothetical world where siamese twins start being born on a regular basis-- a proper mutation, nearly, rather than a freak. This story asks what happens when siamese twins just can't get along in the same body. It is not as silly of an idea as it sounds, and Jackson handles the conceit rather well-- aside from the concerns that I note here. The world building was very well done.)
Three-and-a-half stars, really. In the end I rounded up rather than down for the energy of the author.