Nightmare is not a particularly well-scripted movie. The twists are all obvious. The way crazy George Tatum's doctors track him -- a computer more advanced than we have now -- is ridiculous. Some of the character's motivations are little hard to believe at times and the things they say sound like movie lines rather than real things people say to each other. Crazy George is made the victim of a secret government mind control experiment, which is way overboard; making him simply nuts would've sufficed.
But, that gore. I think the movie's gore effects have such impact because the film never flinches away from them. We see every impact into the victim's body. We're forced to view a close-up of blood spurting from a slashed neck, or out of a puncture wound from a hammer, or from the stump of a neck. It's intense and makes this otherwise not-so-great film quite memorable.
[watched on a laptop in a barn. nothing defeats the Six Weeks!]