I went to see Pope Joan with a group of friends the day it was released in Germany. Most of us had already read the book, but some had not. We were all quite skeptical as to whether the movie would be true to the book and whether it wasn't a little too long (two and a half hours seemed a lot to us!). But, after having seen it, I must say it was one of the most absorbing two-and-half-hours-movie I've ever seen. I didn't have a look at my watch once.
The story is quite true to the book. I read the book at least 8 years ago, so I had forgotten most of it. However, when the plot evolved, my memory came back and I didn't notice any major deviances from the book, neither did my friends.
"Pope Joan" is just a classical historical movie and everyone who loves that genre, will most certainly enjoy the movie! It has everything you need: a charismatic heroine who challenges the role of women in her time; some good and wise clergymen (John Goodman is great fun as food-loving, jovial Pope!); some evil and brutal clergymen who want to spread Christianity violently; some scheming and intrigues; a good portion of war and battle scenes; a romantic love story and, last but not least that interesting question at the end that will certainly cause you to discuss with your friends and search wikipedia: Was there really a woman who became Pope?