While in college, serving on a committee that brought speakers to campus, I was responsible for driving Larry Linville (Frank Burns) and his wife for a few days. You couldn't have met a nicer or funnier guy. Very classy.
A friend of mine says the same thing. He met Larry Linvelle shortly before his death and couldn't say enough nice things about him. For me, Larry/Frank made the show. He was MASH's perfect comic foil. The loss of Trapper John and his subsequent replacement with the weak BJ Honeycutt was too much to bear. Most importantly, though, was the dramatically ingenious loss of Col. Henry Blake. Not everybody made it home from Korea, and his departure was the most honest thing about the entire show.
That is usually the way it is. And the popular characters usually turn out to be pompous jerks in real life.