When Howard Duff was Sam Spade, the performance was 5-star. Sadly, the producers were frightened by the McCarthy "there's a commie under every bed" scare, and fired him. His replacements, including Steve Dunne, were uniformly dreadful. One star at best.
Duff -- debonaire, sexy, charming
Dunne -- not very good
Sam Spade stories were not supposed to be moving; they were supposed to be charming and funny.
There are of course a few remnants of the 50s that have to be overlooked -- like the rampant sexism. Every woman is either a femme fatale, a devil in a sexy red dress, or an airhead. We are all supposed to love Sam's Secretary, who is in the airhead column. Hard to take these days.