I'm going to do this without spoiling the movie for you, because I think this is one of those films you should experience as free of fore-knowledge as possible.
Christopher Nolan has been making the same basic film since the beginning of his career, and one of the things that makes his filmography compelling is the way he circles the central idea in his work.
"Inception," like his earlier work, deals with a broken man, determined to fix his mistakes but only making things worse in the process. That could easily describe "Memento" or "The Prestige" or "The Dark Knight" or even his one remake, "Insomnia." Yet even with him returning to this idea, worrying at it, exploring different ways it can play out, he doesn't feel like he's stuck or marking time. I'd argue the opposite is true: by refining this idea over time and over different films and in different ways, Nolan is becoming merciless in his ability to engage both intellectually and emotionally. As a result, "Inception" flattened me, and even now, more than a week after my first viewing of it, I find myself turning over images and ideas from the film almost constantly.
Shrouded in secrecy during production, the film isn't really built as a narrative shell game with mind-blowing twists and turns so much as it is a logical and orderly descent into a trippy but airtight exploration of the way we frequently chase illusory versions of the people in our lives while ignoring the real flesh-and-blood imperfections that we don't want to acknowledge. Taken as a simple exploration of a marriage that has imploded, "Inception" is harrowing and brutal, and all the SF trappings layered in on top of that only serve to make that stark emotional truth palatable in some way.