There's a reason certain science fiction authors are a Big Deal. Even if one doesn't like them, even if one hates their books or thinks they're mediocre writers, there's a reason society has accorded an author "classic" status over the decades. It has nothing to do with the ability to write or even the ability to create a coherent story. It's all about ideas.
I'm going to be honest here: Arthur C. Clarke the writer doesn't impress me much. Written while he was still impressionable about such things, Childhood's End draws a lot on paranormal and psychic phenomena of which I am sceptical. Although I can't separate my bias from this opinion, I feel that relying on such plot devices also weakens the story itself.
The short length of the book combined with the span of years it covers should tell you everything you need to know: this is more novella than novel (and in fact began that way), more sketch than story. I'll grant that there are one or two well-developed characters; however, overall, there's more exposition and extrapolation than there is action or adventure.
So if this were written by someone else, I'd be content to give Childhood's End a single star and lament its lacklustre writing. But it wasn't written by someone else. It was written by Arthur C. Clarke, and that means something.
This is where you accuse me of kowtowing to fanboyism. (If extreme Twilight fans are "Twihards", what do we call extreme Clarke fans? Clarkehards?) It's silly to give five stars to every book an author writes just because one likes that author; that just distorts the meaning of the rating scale in general. Indeed, you'll notice that Childhood's End has received an average three stars.
Clarke is often renowned for predicting (or inventing) certain technologies, such as satellites. It invariably comes up sometime when discussing the man's life. In the 64-word biography at the back of this edition, for instance, it mentions his invention of artificial satellites and also claims he is considered the "greatest science fiction writer of all time." These statements may be exaggerated, but they reveal the reason why Clarke is a Big Deal. There's another example in the text itself, where Clarke mentions a method of identifying the paternity of children using blood (DNA by any other name would smell as sweet).
He has ideas. Not just one or two ideas, but lots of ideas. In truth, all writers have ideas, and science fiction writers in general have lots of ideas that aren't fit for polite company (hence why we lock them in basements and feed them discarded boxes of pizza). Few manage to articulate ideas on a scale as profound as Arthur C. Clarke does.
Childhood's End is a story of transcendence, of the end of humanity and the beginning of something new. It's a tribute to Clarke's vision that he manages to convey something so startling in a book with such mediocre writing. I didn't much enjoy the cursory way in which Clarke tells us all about how human existence changes as the Overlords shape society. He glosses over any potential problems that would crop up in real life—because that's not the point of the book, as much as many of us, including me, probably wanted it to be. In Childhood's End, utopia is just a side effect.
I enjoyed the juxtaposition of the Overlords with evolving humanity. The final revelation about the Overlords' goals and their tragic inability to follow humanity where it eventually must go is breathtaking. Clarke makes us question, firstly, if this sort of transcendence is desirable, and secondly, if this sort of transcendence is even avoidable. (It certainly seems like the answer to the second is "no," not when an alien species essentially springs it on us unawares.) If we don't destroy ourselves in nuclear war, then we're always going to be asking the question, "What's next? What's out there for us? Where do we go from here?"
Since Childhood's End burst on to the scene, there's an argument to be made (because I would happily make it) that other people have asked this question better, that others have explored it in weirder, more wonderful ways. Maybe we won't reach a deadend or achieve transcendence, the two options the Clarke holds over our heads here. Maybe we're destined to survive, but only just. Maybe cyberpunk is right, and we'll form a symbiotic alliance with our technology.
So I don't think Clarke is the "greatest science fiction writer" of all time, and I'm not even that impressed by his writing. But he holds an important place in the science fiction canon, and Childhood's End holds an important place in his oeuvre. To me, a science fiction book is outstanding if it manages to explore a profound theme while providing an entertaining story. Lilith's Brood comes to mind. Childhood's End isn't quite as disturbing, but it has the same sort of genius. I can't drape it with the same accolades, because it's not outstanding. But it is notable, and it is worth reading.