I really liked this book (obvs). It was more serious than I expected it to be, but still funny; it was a novel about a girl with problems without becoming a "problem novel." While the premise of two girls being able to trick their families into letting them live alone in a beach house with a $1000 allowance is a bit fantastical, the relationships that Mlynoswki created were very realistic.
As many reviewers have noted, a lot of this book is about sex (without there being any sex scenes in it). I read a few reviews that complained about where the book went with it, but I found it very refreshing. It seems like most YA "sex books" are either cautionary tales or, in an attempt to have a sex-positive message, are romantically unrealistic (for example, The DUFF and Not That Kind Of Girl, supposedly feminist YA books, both set up this fantastical story arc where the insecure girl take up a secret sexual relationship with a boy she doesn't really know, and it turns out he's a great guy who just wants to be her amazing boyfriend. They live happily ever after. The end. The way this kind of story is handled in Prep is one of the reasons I love that book. Teenage girls, read that instead!). "Ten Things We Did" has a teen girl who takes sex very seriously, really wants to do it, does it fairly responsibly, but still ends up a little screwed. And the reason that she ends up a little screwed in the end is not because sex is bad but because relationships are hard and no one is really great at them - especially teenagers. And it does all this without - I think - being very "messagey." Also, I can't remember any time I read a book where a teenage girl gets an STD that she quickly and reasonably takes care of. Kudos.
The girl friend relationships in this book are also amazing and spot on and the familial relationships are a bit teary and well done. As a (barely) adult, I think this book captures the feeling we get as we grow up that we are being abandoned and forced into something we are not ready for as well as the fun stuff. Hot tubs! Peach schnapps! Sleepovers! Dinner parties!