I am a huge fan of Cary Grant but this film must be one of his worst. The fact that the plot is totally contrived would not be a problem (most of Cary Grant's best films require a little suspension of disbelief) if it weren't for the paper-thin characterisation, the long-winded dialogue and the wooden acting.
Robert Mitchum is completely out of place, Cary Grant looks bored (that's not just underacting!) and Deborah Kerr and Jean Simmons have both been much better in other films.
Skip this and go straight to The Philadelphia Story, His Girl Friday or The Awful Truth, all of which deal with infidelity/divorce with far more wit and style.