The footage is grainy (it was shot on a DVX100, barf) and some of the conversation and small talk is pretty fake, but otherwise a very good movie. Jerome Bixby was of course a writer of some of the best episodes of The Twilight Zone and Star Trek. He comes from an era in which SF was about ideas instead of 'splosions and merchandising.
It's take on immortality is quite refreshing, particularly pointing out that there wouldn't be any wise, all-knowing immortals. He teaches history but his knowledge is mostly based on study not his own experiences. Though they still can't resist having John know several famous historical figures.
It's a film that really draws you in much like the characters are drawn in to John's story.
The score is also good, too!
The religious bit goes on a bit too long, though. If you've never been exposed to the film's ideas about religion in general and the Abrahamic religions in particular I'm sure this part of the movie is mind-blowing, devastating, offensive, or all of the above. If you're already familiar with it (as I was) I think that part of the movie overstays its welcome a bit.
I also love his "explanation" at the end. Very clever and interesting.
I like to imagine what might happen to these characters in 20 years time. You could easily imagine Tony Todd's character becoming preoccupied with the story and trying to track John down, maybe finding John's girlfriend Sandy.
I make it a point to watch this film every now and again. Really a great movie.