The premise is interesting: two boys who are on opposite ends of the popularity spectrum end up having their lives intersect because of a robotic companion named Rose. David is popular and wealthy while Charlie's family is "off the grid" both literally and not. Although Rose starts out as David's companion (prescribed by the school psychologist as a cure for "dissociative disorder") an unexpected discovery forces her to turn to Charlie for friendship.
The basic idea of the story is fun (it reminded me a little of the movie Weird Science), but this isn't just a story about a robotic sex doll come to life. Cusick turns that idea on its head with Rose giving David an electric shock every time his behavior does not comply with her Intimacy Clock.
The book is very funny and entertaining, though the execution feels a little bit disjointed. While the first half focuses primarily on David, Charlie essentially dominates the second half. I expected, however, that a story about two such opposites would somehow show them interacting a bit more. While their lives do intersect throughout the story, it tends to be in rather peripheral ways. This means that the thread that holds the story together is the robotic companion, Rose.
In particular, I feel David's character is a bit more flat and static than Charlie's, which means that the first part of the book is not nearly as engrossing as the second. While David's character changes little, if at all, throughout the novel, Charlie does change. For this reason, once the story shifts to Charlie, however, the plot picks up dramatically. In addition, because Rose's character becomes less robotic and more human as the book progresses, the relationship between Rose and Charlie is far more complex and interesting than her relationship with David. By the time she meets Charlie, Rose's character has also started to grow and change.
That said, one of the early chapters where Charlie goes on a first-date-from-hell is laugh-out-loud hilarious. Also, I encourage readers to stick through the David chapters because the pay-off once Rose meets Charlie is worth the wait.
A Note on Craft:
In addition to the Charlie chapters being very satisfying to read, this book is an excellent example in terms of craft, in particluar Point of View. These days it's rare to find a contemporary teen book written in omniscient POV, as this one is. You can find it in middle grade novels, like DiCamillo's The Tale of Despereaux or the Lemony Snicket books, though these examples differ from Girl Parts in that they have a strong narratorial presence. Cusick's choice of POV, on the other hand, is more akin to Eva Ibbotson's style where the narrator is virtually invisible and slips in and out of each character's viewpoint so seamlessly, you hardly notice it. Overall, both because of the Charlie chapters and the POV aspect of craft, this book is definitely worth the read.