Grave Mercy was enjoyable enough. Because of a few things, however, I didn't love it.
Firstly, the romance between Gavriel Duval and Ismae feels so forced. I really wish they just had a platonic relationship, not a romantic one. Secondly, the middle part of the book isn't all too interesting. I don't want court politics; I want assassinations. I don't want the court sitting around deciding what to do; I want them to actually do something! The idea of assassin nuns is very cool, but Ismae doesn't actually do a lot of killing.
Other than that, I quite enjoyed Grave Mercy. I like the fantasy aspect, it's clear the author thought out the setting, Ismae is likable enough, Duval on his own isn't too bad, and maybe I'm just not a perceptive reader, but I totally didn't guess who the traitor would be.
Overall a great fantasy read. Just a fair warning: be prepared to spend a day or two reading it. It's quite a thick book.