Suspicion is a great film, as are most Alfred Hitchcock films. It features two talented classic stars, Cary Grant and Joan Fontaine, and is very suspenseful and chilling. Although this film could have been much better had the ending been different, it is still quite good as it is, especially because of Grant's amazing (but much ignored) performance as Johnny.
Essentially, Suspicion is the story of a bookish, shy English girl (Joan Fontaine) who falls in love with a charming but irresponsible man named Johnny (Cary Grant). As the film progresses, the audience begins to suspect Johnny of more than simply gambling and being irresponsible, which raises the question - are the suspicions justified or is "Monkey Face" (what Johnny calls his wife) just being paranoid?
The film progresses, building to a seemingly unforgettable conclusion - but then suddenly, and very unconvincingly, Johnny is vindicated! In my opinion, this ending, while still making Suspicion a great film and enjoyable to watch, really detracts from the overall effect. I feel that Hitchcock's original ending, in which Johnny gives his wife the poisoned milk, she drinks it, but writes a letter beforehand saying that she knew he was going to murder her, would have been far more effective. Sadly, however, because of Grant's matinee-idol appeal, the studio did not allow Hitchcock to cast him as a murderer (they feared it would hurt his popularity).
Anyhow, even though it is frustrating that Grant was so constrained by the studios and by his own persona, Suspicion is still a good film as it is, and is totally worth seeing!