Trapped in his poor solar system for more than two thousand million years, uprooted by endless calamities over and over again, Man has had to move from planet to planet, undergone physically and mentally unbearable misery, given up all his great achievements, changed and re-made his own form, started from scratch repetitively, admired and laughed at himself at once, yet craved for an obscure future, though he is doomed to be destroyed....
On one hand being "classic" in SF could, not unusually, mean "outdated" in terms of scientific prediction. And it is indeed a book full of baseless self-indulgence, as some people put it, if your expectation is in such a perspective.
Even its philosophy, which is the real charm, sometimes sounds a bit more like layman philosophy though the author did take his doctorate in the subject.
On the other hand what makes it really brilliant lies in its sublime tragicality, elegant detachment, ruthless yet heroic aloofness towards the extreme cruelty and extreme beauty of the universe.
No wonder Stapledon influenced generations and generations of admirers, among whose names Arthur C. Clarke, Stanislaw Lem and many others shine.
"Great are the stars, and man is of no account to them. But man is a fair spirit, whom a star conceived and a star kills.... But when he is done he will not be nothing, not as though he had never been; for he is eternally a beauty in the eternal form of things."
"Man himself, at the very least, is music, a brave theme that makes music also of its vast accompaniment, its matrix of storms and stars.... And so we may go forward together with laughter in our hearts, and peace, thankful for the past, and for our own courage. For we shall make after all a fair conclusion to this brief music that is man."
(Also a fuller quotation: http://book.douban.com/subject/discussion/29589854/