I saw Primer on IFC a while back. I had to watch it a second time to get a good grasp on it though.
It was mainly the technical jargon that made it impossible to completely follow. I think I would have gotten most of it if it wasn't for that.
The director has an interesting take on time travel. I like how he showed the little paradoxes that time travel would create and how they turn in to huge messes. Interesting weird unique movie for sure.