I've always been intrigued by what I've read about Primer, and interested in watching it, but have always passed over it. Perhaps it's because I only really watch movies with my wife and she has a love/hate relationship with time travel (though it doesn't explicitly say there is time travel involved, it was heavily implied from the things I read on it).
If she's into time travel she will love Primer. One of my favorite indie sci-fi flicks of the past few decades. Shane Carruth is wonderful. It's a novelistic film, so you'll have to pay attention closely and multiple viewings are almost required. I recommend watching the film with the subtitles on, as the dialogue is complex and jargon-laden and can be difficult to catch on to. It's a rich and complex film, but there's a ton of material online that can be used to help understand the events.