I've had this book on my shelf for quite a while and have finally gotten around to reading it. Overall, I have a favorable impression of it but as a Catholic, some of the slant left a bad taste in my mouth and so I'll have more to say here about what I didn't like than what I did. So I hope this doesn't come off as overly critical.
With the exception of only one passage involving (what I thought to be) excessive name-dropping (or rather, introducing too many new "characters" at once), I thought the book was extremely readable. His style is accessible and lends itself both to speed reading and consumption by small chunks.
I did particularly appreciate the 18th chapter on Christian worship & art. It was as succinct and accurate of a history on the subject as I've seen and could easily be read outside the context of the book. I also appreciated his fidelity to chronology in the ante-Nicene period (after that he disregards any chronological interest like most early Church historians).
It always seems to me like most Church historians have a tendency to rush through the first 300 years (with obligatory treatments of the Gnostic controversy and persecutions under various emperors) so they can get to Arius, Nestorius and the Monophysites. There's nothing wrong with this of course, I'm just personally more interested in the first 300 years than the Christological & Trinitarian controversies of the 4th & 5th centuries.
Now, the task of any non-Catholic historian is to show how moving from point A (the apostles) to point B (the Catholic Church of the 5th century) could not have been a wholly divine movement (i.e. God didn't approve of the move as a whole). It's tricky to do this since most of the important developments during this time period are indispensable to Christianity as we know it. So the greatest weapon here is to cast doubt.
Chadwick uses doubt to his advantage on a number of occasions, most notably when the secular authorities become interested (to whatever degree) in the affairs of the Church. But this approach to me seems about as respectable as Raymond Brown's approach to biblical scholarship (that is, that the more doubt you cast on tradition, the more genuine of a scholar you are). In Brown's case though, his approach often sets him at odds with his own tradition which, whatever else you can say about Chadwick - at least his scholarship is consistent with the Anglican tradition. But this post-19th century approach fails on a number of levels (with biblical scholarship and with Church history). Scholars may cast doubt on Christian developments as long as they wish, but unless we concede that God is only able to accomplish His earthly work in a tidy way (i.e. that God is unable to accomplish His work if men make the circumstances messy), then whatever objections might arise from these doubts will have little persuasive power over clear thinkers.
He never makes the mistake of arguing from silence, but the silence in his argument is noticeable on more than one occasion. Example: early in the book, he calls the notion that Peter was in Rome for 25 years "3rd century legend" but offers no reason why we should accept 20th century scholarship from one who is not in communion with Peter's successor over "3rd century legend" from one who was. I'll take the latter any day of the week. Yet acceptance of the traditional date for Peter's arrival in Rome (circa 42 AD) by no means necessitates the belief that he was continuously in Rome for 25 years. We know he was in Jerusalem in 49 or 50 AD for the council as attested in Acts 15 for starters.
I find it decidedly ironic (and even comical) that he has absolutely nothing positive to say of any pope until Leo (that pope in which Protestant historians abandon all attempts to downplay Petrine primacy). I have to believe that at least one of those 44 bishops had at least some admirable feat or trait. Chadwick takes every opportunity to embellish any perceived negative trait (such as Victor's apparent rash nature). In fact, by a long shot he says more positive things of the heretic Pelagius than of all popes (including Leo) combined and this is not an exaggeration. Chadwick spends several pages defending what he sees as true Pelagianism (as opposed to the caricature which Augustine and the Catholic Church condemned.
But in the end, if it weren't in his job description to cast doubt on the Petrine primacy then I suppose he'd be a Catholic historian and not an Anglican one. He is skilled in his subtlety and comes off far more objective than many historians that I've read and I'm sure it would be more than enough to fool most. But when you quote Augustine (regarding the Pelagian controversy) saying "causa finita est", a student of Church history will have easy guesswork as to what else you ommitted from that famous line and for what reasons.
For these reasons then, (and probably some others that don't come to mind), I would recommend this book to those who already have a good grasp on Church history, but not for those looking for an introduction. The style and readability certainly lends itself to the latter but he makes a good number of subtle errors and unfair embellishments that a beginner in Church history would be unlikely to know.
And in case you're wondering, the line should read "roma locuta est; causa finita est."