This is actually a pretty interesting book, and the three-star rating is not a negative rating, it's a "I have very mixed feelings about some key elements" three-star rating. 
 Anyway, the characterization of the narrator, Sol, is pretty fantastic. What a nice surprise. She was really interesting, a bit impulsive and just all-around a memorable narrator. I quite liked the romance here--this is one of those books that's going to be labeled "instalove," but it's one where it makes a lot of sense for the characters' circumstances. There's an impulsive moment between the two about 2/3 in that was very well-done and appropriate for the characters' circumstances. 
 I had a tough time buying into the world and the alternate course of history which led to the division of most countries into day and night. It's one of those worlds that if you think about it too much, the number of loopholes (which one character, Jean, exploits) that exist, plus the believability of the universal adoption of this system, made it tough for me to believe it. I also had some issues with the way the night people were referred to as "Smudges," and how it read to me as a derogatory term (especially when contrasted with the notion of "Rays") and yet the night people use it casually, like there's no weight to it. It sounds like I'm being nitpicky, but language is such an important thing in world-building and that read as disconnected to me. If I end up writing about this on the blog, I'll expand on this some more. There was also a speech by a resistance leader that gave me pause, but I can't put my finger on why. 
 WIth all that said... If you're looking for good characterization, it's really worth overlooking the world-building issues and giving Plus-One a shot. Sol is a different sort of YA dystopian lead and actually pretty refreshing.