Abstract: Inception is not a smart movie because as far as I can tell the director intended it to "blow people's minds" with a focus on the metaphysical themes. But that debate is old hat. Fortunately, with art you get out what you put in--in garbled form. For me, it is a movie about ideas and purpose. It's a morality play about not living in the past and about choosing what to make of your future.
Inception ends on an intentionally puzzling note. Is Cobb in the real world or not? If it's a dream (almost) anything can happen, so the hypothesis (effectively) can't be falsified. The one piece of evidence that could possibly lead us to reject the theory is the top falling down, but we don't know if it does. If he's not dreaming it is strange that the kids are close to the same age and wearing very similar clothes, that he wakes up without cords attached, that he somehow woke up at all, and that everyone nods at him but no one talks to him, etc. All of this seems left intentionally to draw into question the reality of what we see.
Nolan is drawing our attention to the reality, but he should be hoping that we're mindful enough to draw our attention away, to acknowledge we'll never know if it was real of not--we can't--and that we shouldn't care. We shouldn't care because we don't want to live a life that's "real," we want to experience a certain kind of life: one filled with love, compassion, family, sacrifice, and pleasure. That is the life Cobb finds (we believe) at the end of the movie and the life we should leave the theater intent on creating (not having).
The reality, no pun intended, is that the entire movie is an inception on us. It's a "dream" that helps us to discover the importance of living as opposed to obsessing over trivialities. Much of the movie doesn't make sense (why is Cobb being chased by corporations? where did it start? why do the rules change all the time? and why is the ending ambiguous aside from one hard-to-spot clue?) and we're meant to learn that doesn't matter. We learn movies don't have to be realistic to be thrilling. (They just need copious amounts of violence and explosives. Inception would be a bad movie without them.) We learn that your life might be a computer simulation or a dream, but that doesn't imply it can't be meaningful.
Things matter because we make them matter. The movie is a movie--it isn't real. But we argue about how it "really" ended because we choose to make that important, to make it "real." Inception isn't about metaphysics, it's about ethics. It should be about seeing the world is new ways. It's should be about the power of ideas (our mind) to shape our reality, about how our choices determine what is important and what feels real. We understand on a visceral level that the ending is a happy ending whether it's "real" or not.
Despite the comparisons to The Matrix, Inception is really a counterpoint. The Matrix, like Nozick's experience machine, is about how we live for more than just pleasure (or happiness), and about making the right choices to become the kind of person you want (are meant?) to be. In contrast, Inception shows how what is "real" isn't important. We don't want to be in the "real" world so we can see our "real" kids and have "real" accomplishments. We want to live in the "real" world because the people we care about are there. The real Mal is dead because her projection lacks her vitality, Cobb learns. Limbo (like the experience machine) becomes hell for Mal and Cobb because we need social connections--friends and family--to be happy. We don't need them because they are "real" but because that is how we want to experience life. That's why Cobb needs to be with his kids and why, though the camera draws us to the totem and our frontal lobe draws us to hackneyed metaphysics, we are really being guided toward the inception of a novel idea of why and how to live.