Overall, I really, really liked this book --- I withheld one star for some minor complaints that made it fall short of perfect for me, which I will get to later.
It's very well plotted --- things are introduced early on in the story, in the vignettes capturing the protagonist's childhood on Titan, that all get woven into the plot much later, when he comes to Earth to give a speech at the United States's quincentennial celebration.
It also has great character development; the protagonist, Duncan Makenzie, is a clone, and Clarke does a wonderful job of drawing subtle distinctions between Duncan and his two elder clones (whom Duncan thinks of as both his father and grandfather, and also as much-older brothers ... which they are!) to show that even though they are genetically identical, their lives have given them different outlooks, personalities and skills, even though they remain close enough to guess each other's thoughts and complete each other's sentences.
The character I found most intriguing wasn't any of them, however. It was Karl Helmer, Duncan's best friend in childhood (and lover in adolescence --- apparently this future society is "bi-normative" as opposed to heteronormative, because most of the characters assume Duncan is bisexual and Duncan himself says he finds people who are exclusively gay or straight to be somewhat odd) who fell out with Duncan and hasn't spoken to him in years. Karl is very intense, and also very emotional. He's also a genius at math and physics, and it is in his capacity as brilliant physicist that he re-enters the story much later. You don't often see characters in science fiction who are both highly emotional and also masters of some rigorous discipline like astrophysics --- more often science-fiction writers seem to go with the Coldly Impersonal Scientist trope, or The Scientist Who Loves His/Her Work More Than Anything Else.
The last thing I thought was really clever and wonderful about this book is the two worlds it depicts --- Titan, where the atmosphere is made out of methane and ammonia; and Earth, where civilization has advanced to such an astonishing degree that there's no more violence (even Duncan, the rugged colonist, has never handled a weapon, eaten meat, or killed anything), everything is very safe, and Earth's high-technology civilization coexists peacefully with its resurgent wilderness.
The main thing I didn't like was an odd failure of characterization: the one major female character, Calindy, never seemed quite real to me. Part of this is because most of our first impressions of her come from Duncan's rosy, soft-focus recollections of her from early adolescence, when he and Karl both became infatuated with her, but some of it does come from Clarke's failure to give her a discernible inner life.
Finally, there was one scene near the end of the book that made me very, very uneasy. Duncan was going to get himself cloned, to perpetuate the Makenzie line, and while he's doing that we find that the surrogate mothers have volunteered for that duty because they want to have children. That sounds laudable, but consider that when most women say they want children, they mean they want to keep the children, and raise them. Precious few women just want to go through pregnancy and labor, and then hand the child off to some stranger. (Read Ann Fessler's history The Girls Who Went Away for some corroboration of that statement). To make the ethics of this arrangement even murkier, Clarke gives the impression that many, if not all, of these women are developmentally disabled, which calls into question how well they understood what they were signing up for.
Anyway, it's a terrific novel, well realized, well plotted and well characterized, with only one jarring exception, and one troubling detail in its utopian future society.