Having read his earlier book on the fall of the Roman Empire and being very interested in what is commonly called the "dark ages", I really looked forward to this one, but it doesn't quite deliver.
For a start, the writing style seems to vary from sober historian to slangy joviality (at times he sounds like a historian on speed). The latter doesn't work. You can write pop history without descending to attempting "cool" language or John Cleese jokes (which foreign readers will not understand anyway). There's too much repetition and verbosity. It needs a good editor with guts to tell the author "Peter, keep it simple and try not to be the Nigel Kennedy of history writing."
The book is also very long and looking through the arguments and supporting information you come to the conclusion that there simply is not enough material on the subject probably to justify such a long book. Written material is scant and the archeological remains are not massive either. This means that much of the work is devoted to arguing about migration and invasion hypotheses and using parallels from the modern world to try and construct a model for the movements of peoples. If you don't know about (or are not interested in) the various theories on this subject it can be quite difficult or boring. However, he handles many of the arguments, particularly given the limited material, well and his thesis is well presented. What is weak is the use of comparisons with other ages. He continually uses the example of the Boer Trekkers as the virtually sole example of similar migrations. If his theory relies on comparatives with other similar migrations in history, I would expect to see more examples.
As a history graduate with some prior knowledge I learned a lot from the book, but for the average reader with an interest in the subject, I think that this would be a difficult book. However, the most important take away for me, was that given the scarcity of material and the passing of so much time, hard conclusions as to the whys and hows the Roman Empire collapsed and barabarians took over the remains are very unlikely to be possible. We can certainly see the "what" in our current world (which is why this is written in English, not Latin or Welsh!).