Dark Horse is an exceptionally good read. I think my guts turned inside out with feeling for these characters no less than a dozen times. Kate Sherwood's descriptions of Dan's thoughts and feelings evoke a whole LOT of emotion. I cried quite a lot during the last half of Part 1 (which is the first half of the book - there are 2 parts within it). 
 Kate's bio in the back of the book says that she's driven by characters, and that is clearly evident. The whole book is character-driven, though it's what you see or suspect of the characters through Dan's thoughts. There are some very charged scenes, but they always have a purpose for the characters, not just for the sake of it. 
 Dan's insecurities did rather drive me crazy (because you want him to get over his issues so he can be happy again), but I consider that to be a good thing; it feels real and natural for Dan to feel the things he does, both the comfortable feelings of the traditional dating with Ryan and the uncomfortable of the non-traditional situation with Jeff and Evan. That doesn't mean I don't want him to get past them (read: get on with it), but if he didn't have those thoughts and feelings, it would feel glossed over and would lose some a LOT of the raw emotional impact of the character and the situations. 
 If I have a negative, it's that the 3rd person present POV felt a bit clunky and awkward maybe a half-dozen or more times. Those few times broke me out of the trance of the story and caused me to re-read for clarity. However, for a 375-page book, that's not all that bad, and certainly didn't stop me from continuing to read it as fast as I could. Truth be told, it's probably something very specific to me as a reader, because I read more carefully than most people do. So, I doubt most readers will be affected by what I was. 
 I highly recommend the read if you're looking for emotional drama. You might want to have a box of tissues close at hand, though.