After having just read Dan Brown's Angels & Demons a few months ago, I was super-psyched for the film. It's an enjoyable movie, but since the book was so fresh in my mind, I couldn't help but to keep comparing the two - and there are a lot of differences.
The Pope has just mysteriously died. Vittoria Vetra (Ayelet Zurer), a scientist, has helped create antimatter - which initially would explain how God created the world. It's also very explosive, and has been stolen from her laboratory, CERN. Meanwhile, Harvard symbologist Robert Langdon (Tom Hanks) is called to Rome after an the ancient society Illuminati is threatening to kill four of the preferiti cardinals, the preferred cardinals to become the next Pope.
Langdon and Vetra set out on a wild goose chase around beautiful Rome to find the Illuminatus. The antimatter is hidden somewhere in Vatican City. When the battery runs out, it'll demolish all of the city and part of Rome. The Camerlango (Ewan McGregor), the Pope's right-hand-man, is helping the two find the antimatter and the missing cardinals so that conclave, the process of electing the Pope, can continue.
Spoiler alert: One of the most interesting things in the book is finding out that the Pope was the Camerlango's father, and that he was such a supporter of science and religion because he was able to conceive a child with the woman he loved via invitro fertilization. There is no mention of that in the film. Also, Vetra's father, a priest and CERN scientist, was deeply religious. He was murdered for the "Illuminati" to get the antimatter. The man murdered in the film was not her father.
Aside from some differences, it was an entertaining movie. People who haven't just read the book will probably especially enjoy it.