Not my favorite.
Yes it's "dark", but that's not why. Singh doesn't go in for torture porn, unlike some writers *cough*sherrilynkenyon*cough*, so there's more telling than showing for the icky stuff, which is fine with me.
I just prefer her books that have more interaction from the secondary characters, this one was really focused on the main couple. It's romantic, to be sure, and intense, and it picks up a lot towards the end, but still... not my favorite.
Also, it suffers a bit from the superhero dilemma; this guy can literally move the earth, so a writer is forced to escalate the stakes in order to give him something to do.
Nitpick: I have never seen the word obsidian used so often outside a geology textbook, a copy editor should have cleaned some of them up.