I've never understood the criticisms leveled at Coppola's daughter about her performance as Mary. What was the substantive difference between her and any 19 year old girl you know? The character she portrayed had the vulnerability, naivite, spontaneousness, sensuousness, and emotional honesty manifested by most of the 19 year olds I've ever encountered.
GFIII to me was the most powerful of the three films. The ending left me emotionally exhausted. the silent scream was one of the most dramatic scenes I've ever witnessed in cinematic history. Some of it was a reach, but that could be argued about the previous two films.