They are very popular among girls who dislike reading, because they describe a glamorous world and reading them is very wish-fulfilling. However, they've got plenty of literary references in them that sail over the heads of most teenage readers.
They're more or less well-written, give credit where credit is due, but let's get one thing straight: they are not great American literature or anything.
Nice, normal person: they're all right, but perhaps you just wish you were Serena?
Obsessed and slightly dippy fangirl: nooooo! they're deep! they're high literature!
Nice, normal person: yeah... keep telling yourself that.
Girl 2: Those books disgust me. I bought the first one and threw it away.
Girl 1: Well I am gorgeous and rich so I can read the books without getting jealous.
Girl 2: Hmf!