I recently bought a VHS copy of this film - don't think it's available on DVD - and watched it again several times, after falling in love with it on late-night TV as a teenager back in the 1970s.
The campy fun of watching the two ultimate horror stars of the '30s duke it out in a context of Satanism, melodrama and modern architecture is undeniable. However, there is something more to this film, and once it hits you, it's hard to get out of your mind.
The unspoken back-story is the horror of World War I in Europe, and both the script - full of chilling references to dead men in trenches and the fate of prisoners of war - and the musical score, made up almost exclusively of excerpts from great symphonic works by European composers of the late 19th/early 20th century, evokes the darkness, damage and pain suffered and witnessed by Europeans who survived that war. Both Karloff and Lugosi were old enough to have vivid memories of the first world war, and Lugosi in particular shows a dramatic range that would surprise most 1931 "Dracula" fans.
Lugosi gives a bravura performance as the kind, gentle, haunted and tormented psychiatrist, Dr. Vitus Werdegast, who is driven to vengeful violence by the dreadful fate of his lost wife and daughter. This film is the best evidence, IMHO, of the talent he is said to have evidenced as a Shakespearian stage actor in his native Hungary. His facial expressions are extreme enough to be melodramatic on film, but they would obviously be just right, gripping and moving, on stage.
This movie strikes me as having two realities: one, it's just an unusually good B-Horror film of the thirties, with two great stars; and two - it's a dark and accurate description of what war does to the people who survive it.
(Just for extra credit - can anybody translate the Latin invocation that Boris Karloff starts in this film with "Cum grano salis"? I really, really hope that he's reciting a recipe in Latin.)