There were enough things about the book that bugged me that I'd give it 4.5 if that were an option but it's good enough for me to round up.
First thing that bugged me, it's really nit-picky I know, the author frequently used the pronoun "she" when referring to a generic person.
Anyway, the book is set up so that it goes through the five major personality traits and the facets of each. The author discusses how the traits are exhibited in mice and humans and also why evolution allowed those traits to survive. It's incredibly fascinating, she goes into the chemical details of addiction and depression and gives anecdotes from her own life to illustrate how some of the traits are displayed.
There were two sections that kinda bugged me though. When she was talking about politics, I felt like she propagated the (in my opinion false) idea that there are basically two political parties and one is completely one way and the other is completely the other way. The model she described left no room for libertarianism.
Also, when she talked about religion or religious inclination I thought she displayed a lack of knowledge of the ways in which religion is practiced today. I'm not offended by the idea that my faith is due to genetics. Calvinism certainly would allow that and it wouldn't threaten my belief. However not only did she assert that science and religion are two incompatible things (they aren't) the examples she gave of religious beliefs weren't realistic or at least were applicable only to a small subset of people. I don't assume that every misfortune is divine retribution for some pecadillo, but Holmes seemed to think that's how religion works (it isn't, at least not exclusively).
Despite my minor annoyances, though, I loved reading this book. I love the idea that my personality is a chemical cocktail, a stew of neurons and synapses that caused me to even want to read it.