I'm not linking to any information about Primer because I believe the absolute best way to go into it is with as little background as possible. I'd added it to my Blockbuster queue long enough ago to have completely forgotten the premise, and the DVD sleeve didn't have the usual synopsis. That made it even easier to get fully drawn in and utterly hooked. So if you can, pick it up and pop it in without peeking. It's worth it.
P.S. Just learned that the filmmaker is a Christian and read a great interview with him (which I will gladly share with anyone who sees the film).