This was definitely a tour-de-force performance for Jack Lemmon.
I believe the strength of this film is that his performance allows it to strike a multi-generational cord with viewers...a key theme of the movie is the way the world has changed since they way we remembered it when we were younger, which I think is a pang that all of us get, no matter what age. The world always seems darker and more complicated now and concept that needs no real transposition between eras.
Harry Stoner is a man of his times...swing bands, baseball, and World War II. His life is still influenced by all three, but only in quick flashes of fond memories or flashbacks of a beach in Italy. Lemmon gives us a character that does what he has to do in order to maintain what he has worked for, and that rationalizes that which may or may not be quite on the up-and-up.
Jack Gilford is excellent as his partner in Capri Casuals, representing a voice of conscience that is not quite as blind to circumstance as Jack's character, and there are several other good performances in the film. Jack Lemmon stands out, as he does in most films, and richly deserved the Academy Award for this one.