** spoiler alert **
What a grandly depressing book.
It had some problems with execution, sadly, right around the transition from Part I to Part II. You cannot, cannot, cannot anchor the audience to a character, kill said character, and then expect the audience to simply raise anchor and fix on a successor character we have been given no reason to care about - we connect to the general through the particular.
But once the transition was complete, Clarke fixed the transition problems from Part II to Part III, and from the larger portion of Part III to the hideously, cripplingly sad finale. That is, he connected the successor characters meaningfully to the characters on which the reader had already been anchored.
And speaking of seeing the general through the particular, Clarke also had some difficulties portraying the grand sweep of history. In Part II/III, when human artistic and scientific achievement begins to grind down, he just fails to make it a viscerally moving worry. We never really see how it impacts anyone, we only hear that it does. And given that we do actually see how Karellen's new order benefits the lives of the entire human populace, it's very difficult to sympathize with or see the real importance of the endeavors of, for example, the new Athens colony.
On the other hand, Clarke does a marvelous job with showing the very end. In part, this is because we do see the concrete particulars through George and Jean's slowly vanishing time with their children, but from there he manages to deftly move up the ladder of scope and scale, to a point of civilization-wide abstraction, and show what becomes of all those particulars in a way which, by connecting back to the ground-level originating point of the horror, is directly meaningful to us. We pine for the loss as they do. We see childhood's end.
There were as well some weaknesses in the prose style, but nothing that a bit of editorial polish wouldn't have fixed. I tend to think the standards for such things have gone up over the decades. But when he did have a turn of phrase that was worth keeping, it was striking. For example, "the eternal sunshine of space."
For my part, I was eventually able to identify very well with Karellen, whose own journey, parallel to that of the human resistors', I like to imagine came to a satisfying end. When Jan speaks of his suspicions of Karellen's "plot," I felt a little surge of hope that the Overlords were better equipped to deal with their potential foe than humanity was to deal with them.
I fully see why this is considered one of the paramount works of science fiction.