I was unshure whether to give it two or three stars. Maybe it deserves three and I reacted negatively because I read "Ship-Sister, Star-Sister" - the short story that was to become Starborne, and was spoiled about the ending. But then again... the book offers very little that is not found in the short story, or all the other Silverberg novels, and I didn't like the little things it adds.
I know Silverberg does not write hard SF, but still there were many things that did not make any sense.
The thing that pissed me off the most was the setting. It's on the cover, so it is not a spoiler, just a psychology fail. Poverty, disease, hunger and hardship have been eradicated. Everyone is healthy, rich civilized, cautious, content... what happened? The aliens came and lobotomized humanity?! No such thing is mentioned. Whatever utopists and anti-utopists write humanity will always be curious, and always discontent, especially the youth. No matter how good the conditions are ther will always be people with less than others, people wanting more than they have. Ambition will never die, as long as there are still humans. And while the colonisation might be a good reality show, the idea that it will be the "spark in the lives of humankind" is silly.
Besides, machines were supposed to make enough of everything for everyone by the year 2000, but they did not and will not do so in the forseeable future. And there are all the other small, but annoying details, that do not fit.
/* warning, very minor spoilers ahead */
So you build the biggest spaceship ever and you send it to find a second Earth... and you don't test it first by at least a trial run to the nearest star? How much sense would it need to send ten smaller exploration ships to find a suitable planet, instead of bringing the whole priceless colonizer ship? And if your communication with Earth fails for a few weeks you assume you'll never contant it again, even though you have the fastest ship ever and can go there and back again in a couple of years, after the colony is founded?
The science was bad, the logic was bad, the characters were kind of good (if shallow, except for two of them they were basicly two dementional), but rarely do anything smart. They are a buch of scientists, that do not act like it. They act like reality show actors.
And in the end the book does not deliver its promise. It delivers... something else. Something I already knew from the short story, it was not bad and I give it an extra star for it - but it cannot save the whole book.