This is the only volume of the Handbook series I have so far, but based on it I would seriously consider investing money (and bookshelf space) in the entire set. For hummingbirds at least, this is the first modern treatment of all the world's known species, with illustrations and range maps as well as text, plus a detailed overview of current knowledge on biology, behavior, ecology, taxonomy, evolution, etc. Each species is illustrated field guide-style, with good representation of sexes and geographic variation. The species accounts are short but pithy and include conservation status.
The only real areas of concern, again in reference to the hummingbird section, were in the lack of consistency in the plates, sweeping taxonomic revisions, and editorial bias. Most of the plates are wonderful, but a few are so stylized as to scarcely resemble a real hummingbird, much less the one being portrayed (among the owl plates, too, are some exquisite portraits and some that are cartoonish). Some fairly major revisions of taxonomy - including lumping and splitting of species plus generic reassignments - provide food for thought, but many are controversial and may ultimately be rejected by the ornithological community.
More distressing are the expressed and implied prejudices of the section editor. When published observations disagreed with his unsubstantiated opinion on one issue, he repeatedly insisted that these observations must be wrong. As the editor is European, his opinions are no doubt influenced by his limited field experience with hummingbirds, but it is an abuse of editorial privilege (not to mention unscientific) to use such a forum to arrogantly dismiss the findings of one's colleagues. Moreover, among the 18 authors of hummingbird species accounts I found only a single Latino name and none I recognized as belonging to women, though there are many highly qualified and experienced Latin American and/or female researchers in the field. These are idiosyncracies of this particular editor, and I would not expect these issues to rear their ugly heads throughout the series.