The acting in Demon Seed is fine. Robert Vaughn was creepy as hell as Proteus IV, both sinister and relaxed at the same time, in fact he does an even better job as an evil computer here than Douglas Rain did as HAL-9000 in 2001: A space Odyssey.
The film benefits greatly from an intelligent screenplay, penned by Robert Jaffe and Roger O. Hirson. The dialog is thought provoking and thankfully never really insults the audience's intelligence.
Helming the director's seat is Donald Cammell, who directs with a firm hand and injects a claustrophobic and uncomfortable atmosphere into the film, also managing to maintain the suspense scenes well. Interestingly, he apparently condones the film (assuming that calling the film a 'piece of sh*t' warrants that ).
As others have pointed out there are some dated aspects to the production. The special effects were sparse but thoroughly unconvincing, the only exception being the mesmerizing computer screens that consist of ever-changing shapes and colors that are shown on Proteus's computer terminal. The film takes place in the 'distant' future1995, I believeyet it looks like it's right out of the 70's, no thanks to the cars, clothing, computers and hair styles that give away the films age. This is a small complaint but an annoying one, though it's easy to ignore once you get involved with the story line.
"Demon Seed" is an intelligent sci-fi film that's pretty rare but if you happen to see a copy of it, pick it up if you're in the mood for something smart.