You can sit in your seat, and think for hours, but your results and your opinions seem to differ every time. And after all of the debating and arguing has ended, you'll realize that somehow, in some strange yet fascinating way, this movie personifies your life in some way.
It's very difficult to describe how. The role that Cruise plays, and the plot drawn out for him in the movie, well... they leave something to be desired. It was acted well, though the ending and the final revelation of the true plot seemed to spoil what could've been fantastically surreal, yet just confusing enough that each viewer would draw their own conclusions. Without the last sentence of dialogue, though, the whole ending would have been trash.
"Wake up, David."
As for the personification of it all, it's in the eyes of the beholder. Trauma, friendship, the best sex of your life: they all are present in the movie, but you can immediately identify with what you felt at that moment, when you experienced those exact things. Of course, there are some who can't, which indirectly reminds them that their lives also remain something to be desired.
As everyone else stated, this movie is a very straightforward reminder of the postmodern reality that we live in, though David's "life inside the life" was more intriguing, and brings everyone to the "what-if's" that plague us often in our lives. Questioning reality and life is one thing, but to question it, live it, and recycle it for a born-again you? It's farfetched, but in an enlightning way, it's believeable.