Primer isn't a movie that tries to reach the lowest common denominator in terms of audience. It expects you to meet its concepts. And for those that do there is a great payoff.
Four engineers in a tiny startup realize that they have built something amazing, but don't quite know what they have. Two of them figure out they have built a time machine and manage to buy out the others.
At that point they start going back in time to make money on the stock market, always careful to avoid interfering with themselves or anyone they know. But eventually the temptation to interfere in their own lives becomes too great and the repercussions grow greater and greater.
This harkens back to when science fiction movies were about ideas and not just explosions. There is a lot of tech talk, but is easy to follow and actually has meaning, unlike the techobabble in most science fiction movies.
It isn't boring, but the deliberate pace of this movie may put some people off if they are expecting and action adventure movie. That pace is important because it gives you a chance to catch up with some of the moral and ethical dilemmas this movie sets up.
This is one of those movies that calls for multiple viewings. Each time you can see new implications of the ideas explored in this movie. Although distinct from Pi, this movie has a very similar feel.