I bought this book by mistake. I09 directed me to this link although I had clicked on another book, and after reading the plot summary I decided to get it anyway. I will say that romance is not my favorite genre, and I didn't completely know what I was getting into.
In terms of the SF components, the author gives a sufficient premise to the book that if I don't particularly question or look past the surface, the premise works as well as most urban fantasies. Definitely not hard SF, nor is the book particularly concerned with an SF premise after it is set up. I did enjoy a bit of the twist that the premise gives as our heroes dislocate into another universe, but that was as clever as the SF got.
The rest of the set-up is the standard urban fantasy in multiverse clothing with an emphasis on romance. They are all hot and charismatic, but we are essentially told this, not shown it. They all immediately fall in lust and love, with little development of their relationships, characters, and little convincing world-building. The novel is mainly claustrophobic, with the emphasis almost completely on the obligatory love triangle. It does pass the Bechdel test thanks to the heroine's interactions with her mentor, Abby. Otherwise, I think there was only one other woman in the story who was not a doppelganger of our heroine, and the menfolk handled those interactions.
Tess, our heroine is a hot doctor of parapsychology, beautiful and impulsive, with men absolutely hankering to protect her, but too willful to be stopped as she follows her whims. Ross, Tess' minder and secret agent man with the FBI, is hot, responsible, and utterly devoted to Tess. He intermittently connects (and merges with) his utterly psychopathic doppelganger, making him the werewolf of the piece. And Jake, the hot "echo" who suicided rather than deal with the catastrophe that wrecked his world and ended up with the heroes in their universe, is essentially an energy vampire-- a danger in making contact with any from the heroes' universe, and irresistibly drawn to Tess, who (of course) is utterly delicious. So we end up functionally with kind of a human, werewolf, vampire triangle, with multiverses in which wackiness ensues.
I thought the writing was nice, but I really wanted to see more in the premise, plotting, and characterizations. Yes, there was plenty of panting and moaning for those who read for that, and I think that it was probably good on the romance/sex front (from what I know of the genre). It just didn't give me the depth, wonder, or context I was looking for.