I love Christopher Nolan movies because they make me think. I hate them because he's always trying to show off his intelligence. Inception is one more movie which I will have to watch again to grasp it completely. I think Nolan cuts off a sizable proportion of his audience by making his movies intellectually complex. Many people like me find The Dark Knight amazing, but there are plenty more people who think it sucks and slept before halfway through the movie. (I think that's reasonable - if you just fail to appreciate Ledger's performance and hate superheroes, the movie has little in terms of quality entertainment).
I am meandering away right at the start here, and let me get to the point. Inception is a truly standout movie, purely on merit of its concept, its idea. I remember watching the trailer in May and getting all excited about the basic idea of the movie - robbing your ideas in your subconscious. I couldn't wait to watch it unfold on screen. Other than the basic idea, the movie is good action, with a strong performance from DiCaprio, but that's pretty much it, I believe. The movie could be much, much more.
Watanabe is initially projected as a villain, but gets reduced to a tame supporting role. Ellen Page's role promised much and delivered to a good extent, but did not satisfy. The source of Cobb's disturbed mental state is unconvincing - the whole Mal line could be dropped off and more complex relationships between the team members themselves could be portrayed. I would have liked to see Tom Hardy and Joseph Gordon-Levitt, in particular, with meatier roles - giving their roles as much meat as Ellen Page's role has would be good. Having said that, the Mal line does increase the depth of Cobb's character and does justify Nolan's claim that "the emotional journey of Cobb's character is the driving force of the movie." (That's pretty damn right, Cobb's role to Inception is just what The Joker was to Dark Knight)
Keeping the actors and the roles aside, the development of the idea is really good. The thief creates the world of the dream and the victim fills it with his subconscious. The victim can be made to dream within a dream, driving everyone in the heist deeper into his subconscious and hence closer to his secrets. The deeper the level of the dream, the slower time passes. So, 2 minutes in real life means an hour at the first dream level and 10 hours at the second level and so on. The thief takes advantage of the fact that on waking up, the victim never remembers how his dream began, but only the end. I found the concept of limbo very interesting. It goes like this: If you are hurt in a dream, the pain feels very real, but you are still in your dream. If you die in your dream, you wake up instantly. But, if you die in your dream while being too sedated to wake up, you go into limbo - a state of unconstructed dream space where a few real-time seconds can be decades.
These are the things that make the movie intellectually irresistible - just like The Matrix. And that's why it's a must watch. Watch it seriously with a bunch of non-nuthead friends and you will be talking about it for a while after.