Oh, speaking of things spectacular which are on my website: while you're waiting, check out the MP3 of the Month. More video game music. This one's called The Path of Least Resistance.
Anyway,
I've always (half) joked that I should put together a diagram to figure out the plot, but as it turns out, somebody else beat me to it. Having glanced over it, I think that Primer might be the most tightly-wrapped puzzle movie I've ever seen, mostly by virtue of being about time travel. A few viewings have been enough for me to (basically) understand what's going on without worrying too much over the intricacies of the timeline and its permutations, but whomever put together the diagram, must have slaved over the movie: watched it several times, taken notes, correlated the notes, diagrammed, made inferences, re-diagrammed, made more inferences... all in his spare time. Please shoot me if I ever become this concerned about a movie, (Lynch and Bergman films excepted).
It's insane. No wonder it's so difficult to decipher. Those who are curious -- and trust me, a cursory glance can't possibly ruin the movie for you -- should check out the diagram.