It's not for everyone. I wouldn't say it's a chick flick, because it's not when you define chick flick as a sappy movie where the girl ends up with the guy in the most obvious plot possible. If you want to say a chick flick is a movie with a lead female character, then sure, it's a chick flick.
People who will (or should) like this movie: young adult book enthusiasts, Diablo Cody fans (it's not as prevalent as "Juno," but there's definitely some vocabulary fun when you compare the generations within the film), people who like watching a woman go crazy, 90s nostalgics who want to hear phrases and songs they remember fondly, and general nostalgics who just need a good slap to get on with their lives.
Charlize Theron plays a ghost writer for a young adult book series that is declining rapidly in popularity, despite her claims otherwise. She has severe writer's block while trying to write the last book of the series so she heads home and, like a normal person, attempts to steal back her ex-boyfriend who is happily married and has a newborn. Yes it sounds chick flickish, but the movie is a lot deeper as it explores how people believe they were at their best in the past, hanging onto any flicker of success to get them through their daily routines. Soooo good.
Other pluses: there's a scene that takes place near a shelf full of the YA series she writes and they look exactly like a mix between old school Sweet Valley High, Baby Sitters Club, and the Box Car Children, Patton Oswalt is in it, Charlize Theron is awesome playing a crazy person and her character is the complete embodiment of a character from a trashy YA series in the vein of Gossip Girl and Pretty Little Liars (both terrible, both highly recommended without shame), and it's hilarious despite depressing.
So in a nutshell, it's wonderful. Also, according to her twitter, Judy Blume saw it twice, so that mean's you should definitely check it out.