I'll start with the positive things.
I loved and was intrigued by the main characters from the beginning. The relationship evolved understandably slowly for these two guys with horrible pasts, presents and uncertain futures. From forced partnership to acceptance, trust and finally love. It was realistic, enjoyable and the longlasting sexual tension was handled really well.
I wasn't as kept on the edge of my seat by the other aspects in the story. The Agency seemed a cardboard-cut-out "evil" organization, with 99% of their employees just as blackhearted baddies, barely any shade of grey. No intriguing conspiracy in sight. I guess I had been expecting Manna Francis levels of plot and conspiracy, where the side characters and plot provided suspense even when the main characters were stalling in their development. For the first half of the book I sometimes thought the Agency exists with the sole purpose of making the main characters' life miserable at all times and break my heart for them (half kidding here).
So, the first half was very slow and I think it should be heavily edited. Boyd's repetitive, longwinded musings weren't helping. The second half sped up in many regards and it became tighter and way more enjoyable. New characters were introduced who will probably be interesting in the sequels.
I liked the ending. It's open, but satisfactory enough if you don't intend to read the sequels. I personally will read them, but I need a break now, I'm not prepared for another journey of this length :)