Brilliant, funny, gripping beginning. Dull, interminable, seemingly pointless middle section. And I can say little about the end because I didn't read it. I hear this novel began life as short story. I think I would have preferred that version. Corbell the Rammer was far more interesting than Corbell the explorer. I had been hoping his return to an earth drastically different from today would be as fascinating as Wells's The Time Machine, but it wasn't. The cat-tails were cute though.
I think part of the problem was he didn't have a snappy antagonist in the latter part of the book. Pierce/Peersa, a servant of the State, showed off Corbell's snarky, determined side, but unfortunately those aspects of his character disappeared when Peersa did.
I thoroughly enjoyed the beginning of A World Out of Time (1976) and I'll happily try more Niven, with the hope that they sustain my (perhaps short--but I like to think, picky) attention.
Acquisition note: This was another from my dad's bookshelves. Sorry, dad, I did try to love it!