I read Join via Overdrive last night and found it a wonderful speculative fiction novel. Craftwise, the ending seemed bumpy, but this did not detract from my enjoyment. It is interesting to compare to Haldeman's Forever Peace, which also addresses the idea of a technological merging of identities. There's a resonance with Leckie's Ancillary series, too, in addressing consciousness spread across bodies and awareness of the very embodied experiences.
I was delighted to have randomly picked a book that was so engaging. It probably needs a little trigger warning as death, violence, and fatal illness thread through the plot in meaningful ways.
To compare Join and Forever Peace is somewhat challenging, as the technological connection is fictional, so the impact of the connection on a human's sense of identity can't be said to be more plausible in one than the other. Forever Peace's form of connection does not seem to affect identity nearly as much as in Join. I wonder, though, how "true" that can be. If you sense the embodiment of another, would you still find your sense of identity to be isolated to your "own" body?
Might as well toss the Borg and Voyager's "Unity" episode.