I've only once before been moved to share my feelings about a movie via an Amazon review but this film is a rare genuine "must-see" for all serious cinema lovers. I'd also recommend it for DVD purchase or rental as it is a film that greatly rewards re-watching. In fact, you're unlikely to get all that much of it on a first viewing.
Whether a film should necessarily have to be re-watched several times in order to be understood is an interesting discussion point, but this film earns its reward of our time and attention (it helps that the run-time of the film itself is only 72 minutes). Many people seem to be comparing this film to 'Donnie Darko' but I would make the point that 'Darko' was much more wilfully enigmatic, open to various conflicting interpretations, none of them fixed or 'correct' whereas 'Primer' does have a scientific/narrative 'explanation' albeit one that is difficult to grasp. The two different commentaries (Director and Cast/Crew) are illuminating, another reason for buying the DVD.
I would not classify myself as a science fiction fan but the intelligence and rich characterisation here take this film to a superior level. (The story of the film itself, an authentic "indie" film home-produced, budget financed, edited on the Director's home Mac, is a great tale.) Frankly it defies explanation within a couple of paragraphs and I'm not going to attempt it. Suffice to say it's one of the very best films of the 21st Century so far.