A must see, I would say. This film, based on a true story, presents Christianity in a way that is both credible and amazing, sympathetic and appalling. I'm exaggerating a little, but it is based on real events (which helps with its credibility), at least some of the monks (the ones you see enough of to get a sense for) are very sympathetic people, their dedication to their faith and the people they live among is impressive, and it's hard (maybe not impossible) to imagine people with my kind of secular ethics doing the same thing. Certainly I would not have behaved as these monks did.
I've probably mentioned this before, but when I think of Christianity I tend to think first of the kind of right-of-center political ideology and movement that goes by that name. Next I think of my friends who are Christians but who seem to be just like me except that they go to church on Sundays. Of course, they might not be just like me, but they don't seem obviously more moral or more accepting of life's misfortunes or anything like that. Of Gods and Men shows what I am inclined to think is the real thing, the true form of Christianity. (Although, as a non-Christian, I'm not sure I really have any business passing judgment on what constitutes true Christianity.)