For me, the enjoyment value of this book was all in sex scenes, which were well-written, engaged me in their characterization, and were not repetitive. The spies!plot, I felt, was a bit perfunctory, and several times I was confused about some point or other. In that respect, it was very similar to many other spy romances I've read, whether set in the Georgian or Regency periods.
The hero, Marcus, was fairly typical of his breed: Alpha Male Nobility, often behaving like an ass, but made tolerable to me because of his abject craving for the heroine, which he keeps trying to pretend is a passing fancy that will be swiftly assuaged. The heroine, Elizabeth, is in denial about her lust for him while constantly giving in to it; once she seems resigned to her attraction to Marcus, a dubious spy!plot complication arises as another blocking agent. I had a hard time following their arcs, as the plot was not engaging me, and I skimmed some bits of it.
All that said, I'm still going to read another one of Day's books, to see if I can identify what I liked about the sex scenes aside from the vulnerabilities both characters displayed.