First of all let me just say that I am baffled by anyone giving it 1/10. I have no idea what those people have seen but it certainly wasn't this movie. Depending on your taste in movies I can understand people rating somewhere between 6 and 10. But ratings lower than that seem to come from people that truly hated this on a personal level, for god knows what reason.
The only thing I can think of that would cause people to really dislike Causality is their short attention spans. They seem to struggle to focus on anything that doesn't have some action, care chases, fights, etc. Every five minutes. In a addition they seem to get lost when the storyline is not simple to follow.
When I normally see very polarizing reviews on IMDB, I know I am in for a treat. Most of the time one of the main reasons for this huge divide is that some people just cannot handle slow, atmospheric movies. This movie seems to be no different.
If it comes to the movie itself it is certainly not perfect. But it is a great thriller nonetheless. I love the fact that for the first half of it I was confused about what is actually going on and who are the main protagonists. And this is clearly done on purpose. The best part is that at the end everything makes sense.
The direction was excellent as was acting. It's worth noting that the whole movie was done in one single shot. And this is not just a gimmick as it makes the whole experience very immersive.
All in all a solid 8/10 from me. I was considering giving it 10/10 just to offset those ridiculous 1/10 but I decided to be better than this.