I have to say that for those who have read the book, it's a rather hard movie to enjoy. The liberties taken by the director was far too much in my opinion. I couldn't help but compare the plot and characters (and the events) to what I already know from the book, and whilst not exactly fair, it just couldn't be helped. I didn't like the numerous character, events, and plot changes to the movie, because it removed some of the suspense so vivid in the book (like the death of Cardinal Baggia in the book, but not in the movie).
Tom Hanks was alright, and it was good to see him say his lines while moving, creating a sense of urgency and crisis. Ewan McGregor was brilliant as the Camerlengo, though it would have been nice if it had followed the original plot of him being the Pope's artificially-inseminated son. I don't recall a 'bad' performance from Ewan anyway. He really carried the movie, as far as I'm concerned. As I said, far too many liberties taken. :( The Assassin was a boring piece of turd. I didn't feel his threatening presence at all, which was also rather evident from the book.
In terms of aesthetics, it's a beautiful movie to watch. The shots of the Vatican, St. Peter's Basilica, and Rome are gorgeous. Kudos for that. Pacing was alright, plot development seemed rushed at times, but overall, it was worth watching. Just try to ignore what you know from the book. :P