I got this set for a review in a Dutch birding journal and I can warmly recommend both volumes (Plates & Maps; Species Accounts) to anyone birding in northern South America. Here is an excerpt of my review.
Central and South America (the Neotropics) constitute the most species-rich bird region on Earth. The northernmost part of South America (Ecuador, Colombia, Venezuela and the Guyanas) can be regarded as the core area of the Neotropics and has a stunning 2300 species. Good field guides exist for most of the separate countries in northern South America but now we have a new field guide for the whole area, including the islands near Venezuela (Trinidad & Tobago, Aruba, Curaçao, Bonaire).
This area offers an excellent cross-section of Neotropical habitats, and many birders who first visit, say, Venezuela will probably, sooner or later, also want to visit Ecuador or vice versa. Having more species in a field guide than you need for a single country may appear rather bulky, but this is only a minor disadvantage compared to the benefits of its broader scope concerning distribution and related species, and the possibility of using the same field guide in another country. Moreover, it is worth adding that the distribution maps extend well into Brazil (down to the Amazon river), so this field guide will be of use in northern Brazil as well, although no additional Brazilian species or subspecies have been added.
Restall depicts far more subspecies and plumages than all the other field guides of northern South America combined, and this is a magnificent achievement. The Plates and Maps volume of this new guide has around 6400 bird illustrations!
It cannot be emphasised too strongly how important a good knowledge of subspecies is for birders - particularly in the Neotropics where so many species may be split in the future. This new field guide is of enormous value for this reason, and it may also encourage scientists and birders alike to discover new subspecies.
Restall's Plates and Maps volume at 1.4 kg and 3.4 cm thickness has the merit of being more compact than the existing field guides of Ecuador and Venezuela - particularly given the far greater number of species and subspecies covered. Another significant advantage for the user of this volume is that Restall has helpfully chosen to put all the identification information on the plates themselves. Unlike many other field guides, therefore, the plates include the distribution maps (with an approximate location of subspecies too), plus codes for altitudinal range, status, and abundance, and often a few additional diagnostic remarks, mostly about habitat or habits. Sound descriptions are lacking in the Plates and Maps volume alas, but do occur extensively in the other volume.
All in all, I think that for birders visiting any of the countries in northern South America, this new field guide is the best option.