Great movie in part because of Hanoi Jane. But in large part because of the rest of the cast. Lee Marvin and others, I have noticed lately that lots of movies I really like have a great supporting cast.
The "plastered" non-saintly Kid Shelleen (Lee Marvin) stood before the casket and sang "Hap-p-py Birth-day to yooo, Hap-p-py Birth-day to yooo!" and blew out the funeral candles. It put my 4, 5, 6, and 7 year-old kids, my wife, and me into such a stitch of hilarity that we never forgot it. For those days of Emily Post/Ann Landers' popularity it was so indecorous as to be totally outrageous.
(Not so anymore, I guess. Watching it again 50 years later was not quite as funny as I expected it to be. Today's plethora of deliberately bad manners has made it trite. Sadly.)