I read Hex Hall last year and liked it a lot, but it took Demonglass FINALLY coming into the library to make me realize how much I love the voice in the novel. Sophie feels so absolutely real. I found myself watching to see how Hawkins does it.
Sophie is very sarcastic, almost everything out of her mouth is sarcasm, but she's also very genuine. She wants to have positive relationships with the people around her and her actions show she doesn't want to hurt anyone, sometimes to her detriment, even if her words do not.
I saw myself in her, particularly in the way she relates to her best friend Jenna. I'm way too sarcastic (shocking, I know) and maybe don't always treat my friends the way I want to. I feel jealous of their relationships with other people. I think this is why what I loved most about the book was their relationship. The truest kind of friendship where you can say "You cannot be mad at me too" and the friend understands, and isn't.
For me, this made the book much stronger than the romance. It's seconded only to Sophie's growing relationship with her until-now absent father and the English countryside setting.
Also, notice how what I'm talking about here isn't the magic, the creatures or Sophie's demon-ness? That's the mark of a good fantasy to me. The characters overshadow everything else.