This is a sweet, cute, melancholy, bittersweet sort of book. Lauren Oliver's writing style was a high point for me--it was a distinctive tone, the mood was very well done, it was just effective writing.
It was a very quick read: I started it during work last night and finished it before I went to bed. Of course I did stay up a bit later than usual so I could wrap it up. ;) It is a children's book (Barnes & Noble website is telling me it's for age 9 - 12) but I think it has appeal for many adults.
One of my favorite things about the book was its take on death. The book's "Other Side" was both unique and familiar.
There were some things I could have done without: I felt the last chapter was kind of silly; the very clearly delineated good-vs-evil (which is forgiveable due to it being a children's book); evil being depicted as fat or old; and Liesl telling Po that "You have to be one or the other" (boy or girl), Po setting her straight, and then Po turning out to be a boy in the end anyway... after being called "it" the entire book--I mean, really, in my experience people do not find the idea of a gender-neutral pronoun weird or outlandish and will use one if they know it exists. The first two are forgiveable, the other two are problematic for me.
Overall, I enjoyed reading it quite a bit.