First a small sidenote: I did not actually read this particular edition of the book - rather I read an edition which contained reprints from the collection of the University of Michigan Library; that specific edition is not to be found on the current list of publications of this work on goodreads, however as the book does not have an identifiable ISBN I feel hesitant to add it to goodreads.
The book's strengths lie mainly in the fact that you get some insight into what a guy like d'Holbach, who wrote this stuff more than 250 years ago, thought about various topics. It's very much impossible (in my opinion) to give the book a 'fair' rating, so I decided against rating it. To put it bluntly, the author would if judged by modern standards be considered an ignorant fool (examples of topics covered: humorism (example quote: "thofe who are deprived of imagination are commonly thefe men in whom the phlegm predominates over that facred fire, which is in us the principle of mobility, of the warmth of fentiment, and which vivifies all our intellectual faculties"), phlogiston, physiognomy...), and I'd much rather take medical advice from e.g. an illiterate peasant from, say, Alabama than I would take medical advice from him. A lot of the stuff included is in the 'not even wrong' category, where you sort of find it hard to even know where to start to make the guy appreciate how far off base he is.
He was right about the gods, but you should not read D'Holbach to get a good account about why 'religion is wrong'. The fact that the author had the right idea about some things but was simultaneously hilariously wrong about a lot of other things because of his lack of knowledge is what makes this book worth reading. I believe what the book actually does best is to illustrate in various indirect ways just how far we've actually come in the last 250-300 years.