TCM recently aired this obscurity (with thanks to Something Weird and the American Genre Film Archive for rescuing it).
As a film it isn't much: Poorly shot (16mm) with the camera often fumbling around without edits searching for proper framing, barely adequate sound and a lack of basic of explanation of who is being interviewed and the context of LaVey and his followers.
Still, I found this a lot more interesting than its critical reputation would have it. It's very much a 'time capsule' as others have noted. San Francisco in the late 60s has always been embodied in the public mind by Haight-Ashbury, The Summer Of Love and the Hippie movement in general. LaVey and his Church of Satan fit right in even with their Black Mass trappings. Hedonists and free love folks at their core, LeVay and his disciples simply traded their tie-dyes for devil masks and leather - or, just ditched their clothing entirely (though, still in a mostly sexist manner with naked women being the 'altar' objects).
Aside from the Satanic angle, the other thing that made them public pariahs were their embrace of all types of sexual 'perversions' (LaVey's own words). Interestingly, one of those alleged perversions of the era was homosexuality - hey, they were progressives ahead of their time! Why LaVey got tabbed an underground cult leader was his insistence that he was leading a new religion, as foolish as that was. Indeed, the extensive footage here of the group's rituals leads one more towards mockery rather than fear (although the whipping scene is pretty realistic). Try as he does to make the rituals into a horror show, Director Ray Laurent succeeds more in making them look mundane and more than a bit boring. Heck, the Church of Satan didn't even use live animal sacrifices! What kind of devil worshippers don't spill blood?!
But, even at that the film is interesting: Stripped of all the costumes, nudity and devil scripture this was just a bunch of folks gathered together in some guy's darkened living room - so what if an orgy broke out? SATANIS will never be confused with fine filmmaking, but, it's a valuable document of a time and a place. And, probably the best peek at LeVay as a person -- stripped of all the pomp and circumstance -- and fearsome reputation.