Angels and Demons strips the plot down to basics and ignores the worst parts of the book, and I think the movie succeeds better than the novel. The Illuminati play their part; to my knowledge the history as propounded in the movie is true, or mostly true. I don't know enough how Catholics pick a pope to really say whether all the Vatican insider politics is accurate, but probably.
It's not a great movie, but works okay. Tightly directed by DaVinci director Ron Howard (Splash, Cocoon, How the Grinch Stole Christmas, Apollo 13, A Beautiful Mind, etc) and well acted by everyone. You get a feel for the characters, even the minor ones, and the exploration of religion vs. science is terse and pithy. The action works, and racing through Vatican City seems authentic.
On the Shockwave Radio Theater Scale of 9 to 23, with 23 being top, I'd give Angels and Demons about a 19, rounded up to 20 for showing details of the Large Hadron Collider (which wasn't around when the book was written). Be warned: The movie is violent and icky. On the other hand, you don't have to stay around for the credits: Nothing happens afterward.