I think arguing the complexity of the film is a tad pointless. What makes the film great (albeit not some arthouse masterpiece) is that it holds up to multiple viewings very well as a result of the multiple interpretations available to the audience. There are quite a few more than listed on this thread.
I've met some very unintelligent people who had no trouble understanding the movie whatsoever in the first viewing. I'm guessing they just went with the happy ending. One (fairly stupid) hypothesis I've heard if that Cobb's kids are his totem. So, when he can see their faces, he's in reality.
However, the film is meant to be open to interpretation. The top's spinning at the end isn't the only thing that makes the reality seem dream-like. There's the narrowing corridor in the chase scene early on in the movie.
The film holds up to at least three viewings, which is way better than most big budget hollywood films.
The first time, I just watched it with a completely open mind, and still thought the end was a dream considering how sappy everything was.
The second time, I really tried to see a way where the movie would work if Cobb wasn't dreaming at the end, and still couldn't.
The third time, I took the meta-cinema approach. See, Nolan has said that one of his biggest influences for Inception was Fellini's 8 1/2. Seeing Cobb as the director really works considering that once he gets the assignment, he's much more obsessed with inception than he is with getting home. Ariadne fits the role of set designer, and the guy whose head they are going into is clearly meant to be the audience. Any other character fits a role too, but I don't want to write it all out as my comment length will probably prevent from being read as it stands.
I will say the film is not without its flaws. The corporation chasing Cobb and his crew at the beginning of the film never has any reason to stop chasing them. The Asian guy's company is a rival corporation. So, even given the power he has, a phone call shouldn't be able to call both the police and the other company off Cobb's tail. It's an early plot point, but it's a real shame that the movie just drops it hoping no one would notice, or hoping the Asian guy's phone call took care of it.