Shot in Big Sur on the central California coast in just two weeks and under a very small budget, the film more than makes up for those limitations with an imaginative script, fantastic visuals and well-nuanced acting. William Shatner gives what I consider to be the most subtle, unmannered performance of his career as the protagonist -- a weary, wounded soldier. The succubus who aims to cause his downfall is more than ably portrayed by Allyson Ames, who would've been quite at home in any Bergman film.
Bergman is, in fact, a reference point, with a few scenes obviously inspired by "Persona", "The Seventh Seal" and perhaps "Wild Strawberries". Other influences seem to be some of Kurosawa's early work and even Greek tragedies.
Many people consider the fact that every bit of dialogue (and even the credits) were in the Esperanto language, to be merely a gimmick. In fact, it was an inspired decision, and makes the film independent of time and place; perfectly complimenting the otherworldly mood. Most of the actors do quite well with it, and after a few minutes it sounds natural, and a bit like a cross between Swedish and Latin.
There are a few niggling problems: the actress who portrays the older succubus has a terrible declaimatory style, there are occasionally irrational plot turns, and worse -- the obtrusive subtitles that block out a large swath of the screen. This was necessitated by the fact that only one print of the film survived, and it had had French subtitles printed on it. When the print was rediscovered, director Stevens had to restore it for English-speaking audiences by blocking English subtitles over the top of the French!
I must mention the score, by Outer Limits composer Dominic Frontiere, which perfectly compliments the film. Conrad Hall's cinematography is at times breathtaking -- especially in one scene where Shatner wanders through a field by moonlight, the grasses swirling around him.
The film's denouement stays just on the better side of moralizing and manages to avoid heavy-handed Christian references. Indeed, the statues of Jesus, Mary and various saints in the village chapel seem just as threatening as the demons outside.
Although not quite as morally ambiguous as "The Wicker Man" (and probably shot for 1/10 the budget and in 1/10th the time), Incubus nevertheless is one of my favorite "horror" films of the 1960s, and well worth viewing. By the way, I disagree with the other poster suggested that Incubus is best viewed in a large group. I suggest that the intimate scale of this film works best when watched alone on a rainy night. Prepare to be frightened, disturbed and surprised.
A macabre footnote: within a year, both the actress who portrayed Shatner's sister and the actor who played the incubus would commit suicide.