Sreeyesh Vijayan’s review published on Letterboxd :
Primer is like Einstein's theory of relativity. People would claim that they understood it, but they'd be deluding themselves. It's a uniquely fantastic sci-fi flick, but it's definitely not easy to digest. When it comes to movies that fuck with your mind, Primer would rank in at sodomy.
You'd have to pay good attention to the narration and dialogues to get a gist of what the fuck is happening--this movie does not attempt to simplify the plot. I had to read up on Wikipedia to get things cleared up a bit. Yes, I will be watching it again.
And since now I really appreciate it, I'll just leave this XKCD here: xkcd.com/657/