I've read it and gave my opinion about in an older topic, can't remember which one.
It was interesting to see that someone other than Puzo or FFC had ideas about a new story of The Godfather. I very much like Riesner's idea of the Corleones being involved with the CIA to kill a dictator as that also happened in real life. But I didn't like Riesner's writing style and his development of the story.
Yeah, the ideas themselves weren't that bad. But it was in no way a Part III in resolving the issues from the first 2 movies. Michael only had a cameo. Otherwise it was basically Anthony Corleone going all McBain in Cuba. And when Hagen bites the dust late in the movie no one even bats an eyelash.
One thing I did find interesting was that the burglar scene from Riesner's script was very similar to the one that was eventually used in Part III and the name of a character in Riesner's script was used in the unused Puzo/Coppola Part III script.
