Fantastic, very highly recommended I'll start by saying that it is damn hard to review this film without any spoilers, but I'll do my best. From what I gathered from the trailer, etc., I assumed that this was going to be a film about a man gradually going mad as a result of his loneliness, since in the trailer you see him finding himself and understandably going a little crazy. Surprisingly, this all has a rational explanation which is dealt with within the first 45 minutes.
For me, sci-fi is always a dangerous genre, and should actually be split into 2 categories. Fantasy and drama. There are sci-fi movies like Star Wars set in distant galaxies with aliens and crazy fighting and special effects, and then there are sci-fi movies that deal with 'what if?' questions, such as films based on Philip K. Dick novels, or stuff like Flowers For Algernon. I've never cared about the first category, but I'm a real sucker for the second. This is one of those films.
What's amazing about the movie is that you're really only watching one man (Sam Rockwell) for an hour and a half, but he proves that he has a brilliant range even though he's playing the same character all the way through. Inevitable parallels will be drawn to 2001, what with the monotone computer voiced by Kevin Spacey, but Moon manages to distance itself very successfully by having a computer which doesn't go crazy (I'd like to think that's not really a spoiler). When the 'twist' occurs about half way through, you really feel sucked in, imagining what it would be like to be in that position, and you remain there for the rest of the film. This film succeeds not through fancy special effects, but through the empathy you feel, and that is the mark of truly wonderful picture.
I gave the film 9/10 because although I loved it, I don't feel that it's necessarily a landmark film, and it didn't leave me as drained as, say, 'Requiem For A Dream', but I will undoubtedly watch it again and again. In other words, see it.