We saw Tinker, Tailor, Soldier, Spy this weekend. While we enjoyed it, both of us were still puzzling out exactly what was going on afterwards, and judging by the buzz of the people in the hallway afterwards as we left, we weren't the only ones. Seems there was a motivation missing for the person who turned out to be the mole, unless it was subtle enough that we missed it.
Am considering reading the novel. I understand there were a few elements changed, but not a significant amount.