I got this post-Worldcon in a second-hand bookshop, prompted partly by my current run of re-reading novels I enjoyed as a teenager and partly also by a discussion provoked by rfmcdpei about a month ago. This was one of my favourite Clarke novels then, and I felt it held up pretty well on a return visit. It's a book about Duncan Makenzie, scion of the ruling family of Titan, and his once-in-a-lifetime journey to Earth to attend the 2276 celebrations of the United States (the book was published in 1975, in time for the Bicentennial) and also incidentally to get himself cloned (he is himself a clone.) The good things about it are actually summarised in the subtitle: fantasy, love, and discord.
Fantasy: If there's one thing Clarke has always done well, it's sensawunda, and his visual descriptive passages of the landscape of Titan, the view of Saturn's rings, the gold-lined coral reefs off the coast of Zanzibar, and even the more engineering topics of the Titanic and the asymptotic drive still enthrall me. Of course, his description of some of these have been since overthrown by later discoveries (including those of the Titanic, which led to a much worse later Clarke novel). But it was impossible for me to listen to the real sounds of Titan without thinking of the oxygen flame burning in the hydrocarbon atmosphere. A slightly different aspect of sensawunda was the question of pentominoes, not actually visual description but they did lead to me defacing my school notebooks with peculiar blocky drawings for months after I first read the book.
Also - paragraph added after first posting this - Clarke's vision of the MiniSec units is not at all distant from where it looks like we will be in a very few years when Blackberrys, PDAs and PCs all merge into a single hand-held unit - I'm aware that such things exist already, but Clarke's prediction that they would be practically universal by the early 21st century, and would then stay pretty stable in shape and function for hundreds of years, may well be proved right. What he missed of course was the easy access to information stored elsewhere that we now accept as automatic.
Love and Discord: The setting of the novel reflects Clarke's essentially optimistic humanism. Racism is a dead issue - in A Fall of Moondust, one of the other characters comments on the fact that Duncan McKenzie is an unusual name for a black man; in Imperial Earth, set two centuries later, we don't even find out explicitly that Duncan Makenzie is black until over halfway through (though anyone who's read the earlier book will spot the similarity of the names). In addition, everyone is polyamorous and bisexual, as far as we can tell; and the Duncan-Karl-Calindy love triangle is the core of the plot - yet at the same time we are left in no doubt that the mature Duncan's loyalty remains with the barely sketched Marissa, left behind on Titan. (Yet, of course, at the end of the book we discover that he has disobeyed his elders' instructions and cloned not them/himself, but Karl.)
This was about the time that Robert Silverberg and Ursula Le Guin were writing some of their best work portraying the future of human sexuality, and I think that Clarke, in this book, responded to their challenge much better than Asimov (in, say, his vastly over-rated The Gods Themselves) or Heinlein (who may have set the ball rolling with Stranger in a Strange Land, but whose later works leave me with a strange urge to wash my hands after reading - his book that year was the icky Time Enough for Love). Clarke never rose the challenge as successfully again, and I think Imperial Earth is underappreciated in this regard.
That's not to say that it is without flaws. All the characters seem to be the kind of people you would meet at scientific conferences (or the better class of science fiction convention [whatever that is]). The mystery of What Karl Is Up To has a rather underwhelming explanation - somehow Clarke didn't manage to pull off the sensawunda for me here; the end of the book feels a little rushed by deadline pressure, perhaps. And right at the beginning, the explanation of the Makenzies' genetic problems is clearly wrong - if Malcolm had really suffered radiation damage, it could not have been transmitted to his clone Colin let alone Duncan. But I still like this book a lot.