Note: There are a number of spoilers in this review, so if you have not seen this, PLEASE DO NOT READ THIS POST. You have to go into Timecrimes with absolutely no idea of what it's about to be able to get the most out of it. So, please, go watch the movie and then come back and comment.
I am aware of one time travel theory that maintains that time travel is only possible once a time travel device is invented, and then one can only travel as far back as the point when the device was invented. Timecrimes seems to be employing that theory as the basis for a very clever plot that explores the consequences of interfering with the time stream. It works incredibly well. Why it works well, and not only well but better than perhaps any other time travel movie I've seen, is because it keeps its parameters small. All of the events in the film happen within a 24 hour period (which for Hector turns out to be much longer- heh). There are a set number of variables with which to work and fewer opportunities to get twisted up in the confusing logic of time travel. Certainly it could be argued that (assuming the theory previously mentioned) once you turned on a time machine, it would become a traffic jam of folks from all points in the future trying to go back to the furthest point in time possible. And then Hector has a lot more to worry about than making sure he stabs himself in the arm with a pair of scissors. But it's easy to argue that the time machine operator, having seen the machine's potential for great damage in this little episode, could likely have destroyed it and left the troubling technology for someone else to deal with.
I'm usually a time travel story snob- I can enjoy a good movie or book that involves it as a plot device, but it is very easy for me to spot the holes/flaws in its logic. It's not that I'm super smart- it's just that the very nature of time travel opens up practically innumerable scenarios with myriad causal chains. How could anyone make them all align? The answer, of course, is to keep your goals small, and that's exactly what director Vigalondo does. It's clear that he put a lot of thought into his script, so even if there are minor flaws, a discerning viewer can appreciate not having to do the bulk of the work via suspension of disbelief.
Which of course still leaves us with the question: if Hector 2 got the idea to use the girl to lure Hector 1 to the woods because he saw it happening as Hector 1, then whose idea was it in the first place? But brain breakers like that are what make time travel movies so fun...
You were right, John, I loved it!