Usually I'm in-sync with Christian's reviews but here I have to disagree.
First, Psycho II is vastly superior to Psycho III. In fact it is now considered as one of the best sequels to a horror film. The plot is quite clever with a shocking and at the same time ridiculous and funny last minute resolution. Richard Franklin did a very good job in directing the movie. Anthony Perkins brought a wonderful presence to the screen, first heartbreaking then slowly descending back into madness.
Finally, Jerry Goldsmith wrote a wonderful melancholic score. It helps to sympathize with Norman Bates and make him appear vulnerable. Certainly Goldsmith knows how to write creepy music, especially towards the end with "The Cellar" and "It's Not Your Mother" with really unnerving string sections.