At the risk of sounding like a cultural philistine but this is a movie for only the critics to love.
The admittedly detailed craftmanship (B/W cinematography) overwhelms the premise of the film and bare bones there is no genuine driving narrative. The film tries to drum up suspense for the gradual reveal as to why Mank "betrayed" Hearst/Marion Davies and my impression was there isn't anything there, it's certainly not even the irony of a "Rosebud" reveal. As others have said, the film is not genuinely about the making of Citizen Kane but more Mank's ambivalence with his Hollywood existence/experience, 30's politics, Orson Welles trying to squeeze a screenplay out of Mank, and the fake facade of tinseltown. Yet all these narrative threads seem half assed. They get some "details" right, but I couldn't help but wonder to what purrpose? I will say It is the first time I've ever seen the MGM wonderkind Irving Thaldberg portrayed as a duplicitous weasel.
The title character should be magnetic/charismatic/, Allegedly Hearst loved having Mank front and center at Hearst Castle dinners because of his wit, but as played by Gary Oldman he's one note: washed up, bitter and self destructive throughout the film.
However I concur with all the accolades Amanda Seyfried is getting, her Marion Davies in a relatively brief role is a fleshed out interesting person, and she conveys the ambivalence that Marion supposedly had about her own fishbowl existence as Hearst's long time mistress.
Another distracting note: We get that Mank was this prematurely aging self destructive person boozing his way to an early grave (though he lived 13 more years after Citizen Kane) but having said that, Gary Oldman is way too old looking. When the character states he's 43 , you do a double take. Gary's Mank appears older than Luis B. Mayer, WR Hearst, etc, and when the film flashbacks to 1930, when he's suppose to be a fairly youthful 33 Oldman still looking 63 + years old is just too much cognitive dissonance for me. All the contemporary characters including his wife and brother seem decades younger than him. Even Gary Oldman is not that good of an actor to pull off the illusion.