Well, that's it: I have now read every single novel that has won the Nebula Award.
And while this one is not in the top half of my list, it's not so very close to the bottom either. Most of it is a rather good retelling of Heinlein's Tunnel In The Sky, with better world-building and characterisation. Mia, our narrator and heroine, has grown up on a generation starship where the young folks must endure a month on the surface of whatever nearby planet is handy to become full citizens. Her father, incidentally, is a senior politician on the starship.
This better-than-average sf Bildungsroman is then completely wrecked by the concluding section, in which Mia's people decide to blow up the planet on which she underwent her rite of passage - not because of the brutal treatment meted out by its inhabitants to her and her friends, not because they might be a potential military threat in the future, but purely because they don't use contraception enough. A truly great author might have made this into a great sf story (or at least a satisfactory denouement), but unfortunately Panshin isn't up to it.
So, an OK book with a terrible conclusion.
Did it deserve to win the Nebula? In an indifferent year, it would have been excusable. But this was up against the Hugo-winning Stand on Zanzibar, and also against two books that I reckon are better than either, Do Androids Dream of Electric Sheep? by Philip K. Dick and Picnic on Paradise by Joanna Russ. So, as often happens, a very peculiar Nebula winner, but at least not one that is as embarrassing as The Terminal Experiment or The Quantum Rose.