This is another of those supposedly great works ("science fiction's greatest ascent", according to Stephen Baxter) that is actually a load of pish. Sure, it's undeniably imaginative and even "epic". It's certainly a significant work - virtually none before and few stories since have been written on such a vast scale. But it is boring and repetitive, utterly lacking in humanity (even though it purports to be a history of humanity and concentrates, due to the author's background, on philosophical notions), and feels like it was written by a pedantic Victorian school-teacher.
I can say a few things in its defence though. Even though much of the science is laughably wrong, it is at least pretty good for its time. For example, Stapledon describes plate tectonics (proprosed by Wegener, a meterologist, in 1912, to explain the shapes of Africa and South America and the similar fossils either side of the south Atlantic; not generally accepted until the 1950s or 60s), genetic engineering (although he skips over the details, for obvious reasons), stellar evolution. He is also entirely correct about mankind's utter insignificance on the galactic scale.
But ultimately, it's the boring and repetitive nature of the book that stands out. Not worth reading.
One of my favourite films is The Thing, an adaptation of a novella by John W Campbell. Both the original novella and the film tell their story from the point of view of human protagonists. Why does no-one ever think about how the evil aliens feel? Well, that's what Watts has done with this short story. He's taken John Carpenter's film adaptation (yes, he's working from the film, not from the novella), and re-told it from the evil alien's perspective. And done a damned fine job of it. Highly recommended, and free to read (for now, at any rate) online.
The fifth installment in the series, and series-itis is rearing its head I'm afraid. It's getting a bit silly and over-the-top (you could tell that from the cover: a dude in plate armour, with a Maxim gun to one side and, errm, a nuke going off in the background) but that I can live with. It's fiction, it's entertainment, not serious literature. Unfortunately, there's rather too much politicking and I get the feeling that some fairly important background has been edited out in the process of turning the three huge books that were planned into six small books. That politicking is too opaque to the reader and takes away from the silly entertainment. And there's no chance at all that this would work in isolation - if you've not read the previous books, this will score nul points.
I still got some enjoyment from it, but there were too many points, especially in the last quarter, when I came close to just putting the book down and not finishing it. So I'm afraid that I can't recommend this.