Quite definitely, the best movie ever made. It has a great eeriness to it and the plot is just amazing. It unfolds in an amazingly complicated and intriguing way and none of it really makes sense until the very end. The acting is also exceptional, Bruce Willis finally showing he can act while Brad Pitt is absolutely wonderful as the loony head of the Army. Madeleine Stowe is, of course, her usual talented self.
At this point I have to enter into a short discussion of time-travel...

This movie (without giving too much away) supports my theory of time-travel, if it were possible. The film definitely states that you can't change the past because it's already happened. True, you can do things in the past, but you're not changing it because it already happened the way you're doing it in the past. I know, it's confusing. Back to the Future does it all wrong. Let me explain. Marty changes the past so that his family is completely different. However, even in this altered present, he ended up doing the same thing (going to Doc Brown in the middle of the night to help with the time experiment) even though his circumstances were different. Now, we all know how hard it is to keep a schedule. Little things can have big effects, like you miss your bus so you're late for the dentist, so you have to wait an hour till he's done so you miss your child's/parent's/sibling's recital or whatever. This is like chaos theory. There is zero probability of Marty being in exactly the same place, in exactly the same time, doing exactly the same thing and wearing the exact same clothes, etc as the first time he did it. This means, of course, that either 1) he won't go back into the past at all this time, thereby causing a disruption in the space-time continuum or 2) he'll do something different in the past (a certainty since all his actions were based on his parents having a different history than the one the alternate Marty's parents have) thereby causing the same results. I hope you follow me...
On the other hand (and don't laugh) Bill & Ted had it right. Time is all one piece. Therefore, when they decide to go back in time to steal Ted's dad's keys and place it behind a nearby sign, the keys are there even though they haven't gone back in time to make that happen yet. So actions they will take are already influencing the present. This must be true for time travel to be possible, otherwise the space-time continuum would collapse altogether.

In a nutshell, then, that's one of the things I love about the movie. It gets time-travel theory exactly right. Of course, there's a bit of destiny involved in the movie (see the theatre/disguise scene) but otherwise...

It's impossible to describe the film, really, without giving anything away so I'll just say: If you haven't seen it, go and do so now. If you have, take a look at some of the links below...