Will occupants usually survive a decent autorotation? When I learned to fly helicopters, I of course spent significant time learning about and practicing autorotations.

The CFI at my school, who had around 15,000 hrs (that's right, fifteen thousand!) said a few times that practice, knowledge and currency are vital &mdash; but as long as you got the entry right (following which you can fly to the ground) and executed at least a decent attempt at the flare and cushion, you would survive. Is this a correct take on the survivability of an autorotation? Do fatalities arise primarily from errors during the autorotation entry or flare?

While you might not get to use the machine again, and you might spend some time in hospital, you would live to fly another day.  I am assuming a reasonable place on dry land is available to finally come to rest and no post-crash fire.
If the pilot flubs the flare and landing, the result is frequently survivable.  The reason is that even a flubbed autorotation will reduce the vertical speed.  

Student and instructor walked away from this one:

http://www.nydailynews.com/news/national/helicopter-crashes-westchester-county-airport-article-1.1560222