Trumbo was a skilled hollywood writer who wrote one of my favorite movies, Spartacus. He also was a communist, who went to prison for Contempt of Congress, and then was blacklisted by the film industry for his beliefs. The movie portrays him as someone willing to take a fall to defend the First Amendment, but in reality he was someone who supported some hard line communist dictatorships.
Trumbo managed to break the Hollywood Blacklist by writing under fake names, while helping some of his fellow blacklisted writers to do the same, which is what the movie is about.
As a history lesson the movie is really good. The acting is subperb. The entertainment value of the film isn't exceptional, but it is good.