It's an awesome book either way, but it can be read very much either in a hopeful vein, or a very depressing one:
(Spoiler warning; for various works for the rest of this comment - assume that if I mention a work, there'll be a spoiler later in the sentence...)
Humanity stagnates before ending as the last of humanity's children transforms and are isolated from their parents. The parents generation slowly or not so slowly die out and no more children are born. The transformed children become one single collective intelligence, and that too is subsumed into a greater mind. In the end, the last actual human stays on an earth that evaporates.
On one hand humanity has "ascended" to a higher stage. On the other hand what is left bears no resemblance to humanity.
It belongs to a peculiar and I think quite rare strand of scifi where humanity abandoned humanity and was in the end entirely gone (as opposed to e.g. post-apocalyptic settings where there are still humans left, and where most of the stories tend to end on a hopeful note for repopulation), set in the near-ish future (as opposed to e.g. books like Stapledons Last and First Men, which also describes the end of humanity, but where the end is set millions of years into the future). I don't think that strand is seen very often any more.
There's been plenty, both then and later, where Earth has been abandoned and forgotten by a human diaspora that lives on elsewhere (e.g. Asimov's Robots/Empire/Foundation universe goes there), but that tends to have a very different feel to the stories where humanity just "fades away" or transforms.
My other favourite equally definitive end would be Clifford D. Simak's "City" which is told in the form of legends told by talking dogs of how humans left to transform into something else and eventually disappear.
Another, of sorts, would be Clarke's "City and the Stars", where there are still humans, but only two cities left on Earth after everyone else left and likely ascended. That difference, though, gives it a very different, more hopeful feel (but the last few pages of City and the Stars is the only book to have ever given me the chills), similar to the post-apocalyptic stories of small groups that endures.
Many others have gone with the "humanity almost wiped out, but not quite, oh and here's redeeming factors" theme, such as the various incarnations of Hitchhikers Guide to the Galaxy, where one becomes increasingly callous about the repeated destructions of variations of earth as the series progresses to the point where it's more of a regular event to cheer on safe in the knowledge earth will pop back into existence somehow sooner or later (even Douglas Adams attempt at getting his publisher to shut up about another sequel by ending Mostly Harmless with the destruction of Earth in every reality in the entire multiverse and then going ahead and dying on us didn't stop another sequel from being written ("And another thing..." by Eoin Colfer; surprisingly good for falling in the "lets try to repeatedly murder this story with Vogon death rays" category)).
Titan A.E. is another of the modern dark-ish depictions that gets close to finality, where humanity for most of it is depicted as just a few scattered remnants, but there is hope and not a definitive line drawn under it that in the end makes it quite a hopeful story.
Battlestar Galactica (both incarnations) falls in a similar category where the impending doom starts, gets nasty, and then you reach a steady-state of sorts (though the regular updates on the dwindling number of colonists does help to keep it dark), but then is never drawn to its conclusion, and that fails to make it as solemn.
There are of course other "nearly but not quite the end" stories.
Iain M Banks last Culture novel - The Hydrogen Sonata - has a similar theme, but set on an alien planet where the civilization has decided to ascend. It's a great book, but it too doesn't feel as dark, I guess, because it isn't the end of humanity.
The solemn feeling that this is the end, and not an end set in a very far distant future, but not that far from now is present in Childhoods End and City. The atmosphere in those are also aided by describing the aftermath, including descriptions of the pain of many of those who feel left behind.
I'm sure there are others that I should remember, but I'm too tired to think of any right now. And I'm sure there are some I'm entirely unaware of. I'd be very interested in similar stories, and especially if there are any modern ones that are as "final" (as in: conclusively end with the end of mankind in the near future, instead of copping out with the existence of some small remaining group, or dragging it out into the far future).