This is 'Primer' for dummies.

The movie frustrated me from time to time because of massive plot holes and bad writing. The idea is challenging, and complex enough to have fun with, and not too complex to get lost in. However too many things did not add up.

For such smart kids they sure were reckless and rushed everything as if they were for the first time in a chemistry class. Every guy who has even the slightest idea of timetravel knows that messing with stuff in the past is going to get you in trouble (butterfly effect) and if you are going to do something you have to plan it out carefully.
There are also multiple occasions when they travel back in time for a 2nd time to the same time and place, but do not meet their 'earlier' time traveling selfs there. This was especially awkward with the kiss scene. Arguably the most important scene of the movie. Even if the device somehow canceled out earlier jumps to the same time, they didn't have to interfere but just had to jump and wait.

For the story it might have been much more interesting if they didn't focus so much on the main actor during the final act of the movie and prevent such a boring cliched ending.
There is more, but lets leave it with this

There is 1 interesting detail in the ending though; the papers David throws in the garbage bin after the talk with his father date 2025. I wonder what that implies....
Still, there are way worse movies out there.

Edit: there is another thing I really liked about the movie btw. It used a track by Mark Sixma. :D