It's entertaining enough, but a far cry from Tolstoy and yes, many of the leads are woefully miscast. Especially Fonda, but also Ferrer, Lom, and Homolka. Audrey Hepburn is rather good, though.
Vidor's adaptation suffers from a script that was rewritten during shooting, bad casting (Henry Fonda as Pierre?!), chaotic working conditions in Italy, and a final cut that was not Vidor's. Yet the film remains an impressive condensation of Tolstoy, filmed with a clarity and grace missing from the often bombastic and vulgar Russian mega-production. I wish Vidor had Bondarchuk's budget and running time.
Did they tried to turn 'War and Peace' into a new 'Gone with the Wind'? This would explain a lot. The movie script is disappointing and the sound is terrible. Audrey Hepburn is the only good thing that happened to this movie. I suggest you read the book and watch 'Voyna i mir' (1967), a 4 part epic Soviet movie.