Although I see the idiots over at Entertainment Weekly deemed it a "sci fi cult classic piece of crap", this once lost slice of smoky gothic horror from 1965 is both far from sci-fi and crap. I can't really say it's a "cult classic" though since virtually no one saw it for decades. I picked this up today for $2 on DVD and it's well worth that and than some.
But after finishing it just now, I struggle to immediately hail it as a total existentialist creeper grand slam. The first half is quite meandering with long passages of little dialogue, but I guess that's par the course with an "art house" film. At the same time, I don't think the filmmakers set out to create a film that was meant to exclusively play at small dives with makeshift screens as ever-so-hip intellectuals sip jet black coffee while spouting derision, therefore; I can't give an "artistic" pass to it's sometimes listlessness and rough patches. Much like Herk Harvey's Carnival of Souls. The second half (of just 78 minutes) does deliver a potent blend of chilling B/W imagery and moral questions, both from the good and those on the side of Beelzebub.
The whole shot entirely in the "fake" language Esperanto thing is definitely odd. This is the first time I've ever heard the language spoken and it comes off as a hybrid of Portuguese, French, Italian, and German enunciation all at once. Some of the delivery by the actors comes off as rocky, but Shatner manages to hold it together and not particularly sound like himself. Overall, it's worth checking out, but it's not going to be everyone's cup o' tea. Read about "the curse" the film supposedly carries here, which might be about 90% as interesting as watching Incubus itself.