Compared to its most obvious competitor, Geoff Sample's Collins Field Guide, this guide has recordings of more species (200 v 153ish), almost twice the number of pages in the book, illustrations, and yet is cheaper. The recordings are isolated, not synthesised into a soundscape as the Collins Guide and are not announced. On the whole I think this is the better method, though both have their merits. The auther has included painstaking attempts at transcriptions of the songs & calls. I find them difficult to read, but they do seem to be a pretty good attempt at actually getting the vocalisations down on paper.
Why not 5 stars? Well, though this is a very good guide, it is certainly not "the only field guide you will ever need" as it claims to be. The species accounts in the book are actually surprisingly good, but only species which appear on the cds are described, so for example, Tengmalm's Owl is included but Short-eared Owl is not. This compares to the Collins Guide where (British) species which are not present on the cds still get an account in the book (though this is limited to a description of their songs/calls). There are a few species which are pretty irrelevant from a purely British point of view (Grey Headed Woodpecker is another), but this isn't just a British guide, and almost everything you are likely to hear in the British countryside is present.
Some of the songs/calls sound a little 'different' to my ears, and I wonder if they are recordings of European rather than British birds. Dave Farrow is certainly active all over the world, and the other contributors all have rather 'continental' names.
On the whole though, an extremely good guide and good value for money. I have no hesitation recommending it.