Also, This:

If the films in this list were arranged in order of ascending awesomeness rather than chronologically, Primer would still occupy the final slot. Made on a budget of $7,000 (seven! thousand!), Primer is one of the few movies I have ever watched twice in a row—and certainly the only movie I’ve ever watched at 8 a.m. after having watched it twice in a row on the evening prior. It’s like a deep-tissue massage for your brain—afterwards you may hurt like hell, but you’ll also feel strangely invigorated.

—Mindfuck Movies by Matthew Baldwin

I finally saw Primer a few months ago, and yes, I watched it thrice. Plus jumped around a bit watching sections, to see if I'd figured it all out completely and where I tripped up, a fourth time. If you haven't seen it yet, go, now, get your hands on a copy. And be prepared to spend the rest of the weekend watching it again and again. The rest of the movies on Baldwin's list are favourites, too. About the only one missing that immediately comes to mind is The Fountain. But of course that may be because I believe (Hugh Jackman + Darren Aronofsky) x Clint Mansell score = WIN!