This is a hard one to rate, because I suspect there's an "it's not you, it's me" thing going on. I felt very little sympathy for the protagonist. She is a Mean Girl, a minion of the most popular girl at school, who gets stuck in a Groundhog Day-like cycle, repeating the last day of her life. 
 In theory, this means there could be opportunity for a great, affecting redemption, but even by the halfway point, she was just still too... I don't know. Mean Girl-y. Shallow. About halfway through the book, I wondered why I was even reading, because I still didn't care about her. But, I did keep reading. It is a compelling premise and it is well-written. 
 I suppose I should credit the author more for choosing this less sympathetic POV. There'd have been greater immediate sympathy for an underdog, for sure, but she instead chose to tell the story of this patently mean, unlikeable girl who runs with bullies. The mean girl is trying, sure, but is it enough? (Maybe that's part of the message: one day is never enough.) I suspect your mileage will vary on this one, depending on your age, where in the social hierarchy you were in high school, or just how angry bullying makes you (for any reason). 
 I also thought at one point that she might have laid the groundwork for an interesting spin on the Groundhog Day idea ((view spoiler)[what if poor Juliet was repeating the last day of her life, too, until Sam saved her? (hide spoiler)]), but then it didn't happen. I liked my version of the story a little better. Don't you hate it when that happens?