I'd classify this as a hard science fiction novel with romantic elements of both the M/F and M/M variety. It has moments of brilliance that kept me reading, along with a mystery which unravels slowly. Be prepared to have just as many questions at the end as in the beginning. It deals with a future Earth, interstellar travel, wormholes/foldspace travel, and time travel. There were points where my eyes glazed over, because I'm not so much a hard sci-fi reader, but getting through those parts that don't appeal to me was worth it for the great character stories, the mystery and the romantic elements. 
 If you are a romance reader (I notice some have marked it as M/M Romance) be advised there is no HEA or HFN, but there is a second book where the story continues...and I'm off to go read that now.