Many other reviews here have outlined the strengths and weaknesses of the film re the truth about the making of Kane, and the relative attributions of credit, blame and opprobrium. I'd like to inject a good word for Roy Scheider's portrayal of RKO boss George Schaefer: His character's struggle to find the right balance between keeping his east coast money men happy, his obvious liking for Welles and the desire to make good movies is very well portrayed.
Something I really enjoyed was the portrayal of Welles' and Mank's visit to the Hearst Castle at San Simeon, California. That is a fascinating place, which saw so many famous and talented people visit during Hearst's time there. There's a movie about the lifetime of that place to be made by someone, though I don't think anyone has ever attempted it? Apparently they didn't use the real location for RKO281 - a lot of it seems to have been done in London. Was that cost, or did the Hearst Castle trustees refuse....? Anyway, if you're up for a good tale woven around some known facts, but not sticking to them too tightly, take the RKO281 ride, you'll have fun. Just don't let it become your true picture of Mr Welles, Mr Hearst or (most of all) Mr Mankiewicz.