Well crafted science fiction story that has held up extremely well throughout the years. I'm keen of Clarke's writing style and the plot devices he uses.
Since there really isn't much bad about this book, and I'm sure all the good that can be said about it has been, I'll focus on what I didn't like, albeit there's not much.
All the characters are extremely bland and undeveloped; not much of a shocker since this is a plot-heavy big-idea sci-fi story. So, it didn't really matter, I cared about what would happen next rather than what the characters were doing. Although it would have been nice if the Overlords were at least made a little more distinguished in their characterization.
The scene with the Ouija board, or whatever the hell they were using, was...weird? Stupid? Extraneous? Choose the adjective as you see proper.
The ending, which I won't spoil, while it wasn't bad it was still kind of unfulfilling and not really explained or approached. No one challenged what happened in a direct or a philosophical sense, they just let it happen (and by they I mean the author). I suppose the Overlords had made them too soft.
Anyway, well worth the read.