The story, director Daniel Manns direction and the cast all comes together in a neat little package. The story is EC Comics grisly even down to casting the good rat as white, the bad rat as black. Daniel Manns direction has a slick and economic polish. And, although Mann never goes for all-out shock, there are some modestly effective scenes of people being overrun by rats.
The cast are all good including later-to-be Clint Eastwood girlfriend/co-star Sondra Locke as the love interest and Bride of Frankenstein herself Elsa Lanchester as the oppressive mother. Particularly good is Bruce Davison who gives a great performance of plausibly sympathetic introversion and psychosis. Even Ernest Borgnine turns his usual simple-minded on-screen presence towards some good.
The best performances come from the rats, particularly Ben, who plays with a disturbing intelligence. In some scenes like where he sniffs out a box of poison and then reappears at the meal table with his eyes narrowed to tiny slits you would swear there was a calculating intelligence inside. He proved so popular with audiences that he received his own sequel, Ben (1973). Remarkably enough, Willard was produced by Bing Crosbys production company, which made a handful of other horror films around the same time.
The film was later remade as Willard (2003) starring Crispin Glover in the title role.