I loved this little gem, and hate having to bring up Primer, because the two films are so differently focused. Primer is written by someone strong in the hard sciences, and wants time travel to be as plausible as possible, while also layering in all the effects of the 'science'. This movie comes right out and says it's most concerned with the ethical implications, and doesn't go into lengthy physics or mathematical explanations.
I finished this the first time and went right back to watch it again, to chart out the timelines using newspapers and clothing as clues. It was as fun the second viewing as the first! While this one ends the way many scifi tales about time travel should, it zig-zags through several different reasons why it chose the ending it did. If, on first viewing, you feel a sense of disappointment (and slight confusion), immediately rewind and watch it again. You'll see why it played out the way it did. A sense of relief should be the result of that second viewing, because, like the characters, you experience time in that world and what the ramifications of all the traveling were.
If you like exploring causality in an easily accessible way, but were put off by the intense scientific gymnastics of Primer (which is fantastic in its own right), check this one out!