Based on some of the other comments I was expecting to hate this movie. I didn't, but I see why the reviews seem so negative. There are so many things to pick at, and no real bright spots to talk up.
It really wasn't that bad. It wasn't great, but it's worth a rental. I'm not sure I would have felt I got my money's worth in the theater.
I'm sure it was a tough chore to convert the original story to a 2-hour movie script. They had to take several liberties and condense quite a bit to do so. Unfortunately, that affected the flow and the movie ends up being a little choppy. How and why they go back in time is pretty well cut out of the movie and without any detail the believability is nil. The story boils down to a pretty standard chase and explosion thriller set in the middle ages.
Definitely treat yourself to a read of the book the movie is based on. Like any book, the depth of characterization is so much more rich and interesting than what comes through in the movie. Once you get past the introductory technical jargon to set up the story it is a real page turner.