This novel was really slow-starting, since there was a LOT of world-building. Most of the world-building was fun after you got over the initial confusion, however. I felt like it balanced itself out by the end, so I'm giving it 4 stars barring a re-read that changes my mind.
The novel centres on Don Rumata (or Anton), an undercover operative on a planet with mediaeval-level of technology from a future Earth. There's also mediaeval-level sanitation, so the authors spend quite a bit of time illustrating the innate disgust that a modern person would need to overcome were he or she actually a resident of a mediaeval town. And the psychological conditioning such a person would need just to be able to eat the food.
I'm not sure I buy the assumption of the inevitability of civilization that is presented, although I'm not certain that the arguments for it were sincere. They seemed to be countered by arguments against giving the god-like Earth technology to the mediaevals. Actually, in retrospect, I'm not sure whether Rumata's mission was to prove their historical theories or enforce them. He mostly spends his time rescuing intellectuals (when he's not maintaining his cover), and the bulk of the plot centres on rescuing a man called Budach whom other agents have mislaid. I'm just not sure what this achieves. [This is where a reread may help.]
I do like the description of Rumata as a god in human form, and not just because of his sword skills and battle tactics. A modern human would have so much broader of a perspective of the world that god-like seems apt. In fact, Anton as Rumata has to keep reminding himself that the people around him are human, or at least, will be
human (since they are so petty and narrow-minded). It's a nice touch.
Update (2015-02-11): The reread didn't change my mind about the rating, although I was tempted to bump it up half a star. The reread also clarified things a lot from the beginning. I may even read it a third time, eventually. I do still feel that those two dialogues near the end are a bit of a weakness because the book relies on them so heavily, but they're also fun to read.