Very enjoyable light entertainment. A crime story that revolves around a clueless but persistent woman. The crime itself isn't all that important, and the focus is not on the narrative drive forward (which is good because it's kinda predictable). The focus is instead on the character of the female lead. Oddly, it's not about her growing as a character, or even about her getting more and more confident about her chosen line of work. It's not even about proving herself to everyone that she is capable.
The story is about persistence, about how this character is somehow ideally suited for this situation, but just lacks the knowledge to be truly successful.
If this wasn't such a light and airy movie, I would suspect that it was a metaphor for living in the age of the Internet, where knowledge is a commodity and anyone who has access to specific areas of knowledge can be an expert. Alas, it's a little less than that.
Unfortunately for the movie, the lack of character depth and the cumbersome box-like production make it seem very TV-ish. Like this was the pilot episode of a series. It also suffers from useless-narrator syndrome. I didn't read the book, but I suspect the source material may share some of the blame for that via lazy exposition.
Speaking from a guy's point of view about what is essentially a chick movie, it doesn't hurt that Heigl is hot, has a nice smile, and can handle a gun. And is a pretty good actress, sure, yeah... that. The casting could have been a lot worse. I would watch Heigl in just about anything if she stayed brunette.