It's said that he did see it - twice - but his reaction sadly isn't reported. Chaplin's biographer Jeffrey Vance relates that

Adolf Hitler was disturbed when he heard Chaplin was at work on “The Great Dictator,” and there is evidence that Hitler actually saw the film. According to an agent who fled Germany after working in the film division of the Nazi Ministry of Culture, Nazi authorities procured a print and Hitler screened the film one evening in solitude. The following evening he again watched the film all by himself. That is all the agent could tell Chaplin. In relaying the anecdote, Chaplin said, “I’d give anything to know what he thought of it."

The nazi regime most certainly had a view, banning it immediately. Hitler's copy is said to have been brought in via Portugal.