As a pocket guide and quick reference, you cannot beat Stokes' "bang for the buck." It offers most of the basics that most birders would need in the field:
* Quick color tab index
* Large photos (which usually capture indicative plumage)
* Good all-up glance at ID, feeding, nesting, and other relevant behavior
* Nice North American habitat guide that zooms in if the range is geographically smaller
* Habitat type info has been very helpful

Downsides are few, but:
1. I think the anatomical reference is incomplete, but for most purposes basically adequate.
2. The "quick reference to the most common birds" pages seem somewhat pointless because the species are so arbitrary.
3. The color quality in some photos aren't as indicative as I'd like, but you have to expect some inconsistencies due to the nature of photographs, habitat and lighting.
4. There really isn't a section on responsible birding, ethics, and avian conservation. Their conservation section consists of BBC and CBC census data only.

That said, I love this guide. Between the Stokes and Sibley ID guides used in tandem, most people won't need another field resource for visual field identification.