I'm one of those few people who completely hated Primer. I found it to be masquerading poor narrative as intellectual mystery. It's possible to present an intricate, complex logic puzzle as story without being totally incoherent, but Primer doesn't even bother.

For a movie that matches Primer in everything it does right, but none of what it does wrong, check out Timecrimes (Los Cronocrimenes). It is far superior.