I enjoyed it, would probably give it an A.
My theory is that everything is a dream and that some of the flashbacks are real. Like moments at the beach or with his wife. I think the whole thing was an elaborately constructed inception for the benefit of Cobb, who is forever stuck in limbo, in order for him to believe that he was returned to the real world to go on living with his kids. The biggest problem with being stuck in this world is that you know it's fake. So if you can really plant ideas, wouldn't it be ideal for Cobb, if he were really stuck in limbo, to implant ideas to make one of the dream layers in to a reality layer?
Sort of how the main character in Memento constructs a new killer to go after, Cobb may have spent enough time in this limbo to slowly construct this elaborate convolution of dreams in which to convince himself he was back in the real world. Basically performing inception on himself to alter each iteration. So in a previous iteration he may have implanted the idea of the children, but not the details so he couldn't accept it, so he creates another world that gives him a reason for not remembering and a method for him to be reunited with them.
I just thought the real world had too many dream elements in them. The passage of time, the sudden travel to far away locations, jumping from 2nd story balconies with little effect, the almost magical vague machine to get them in the dream state where some kind of cable is attached but you never see a needle or blood, ect. In fact, as I was watching the movie I wondered if the real Cobb was some skinny dweeb who created an idealized Leo DiCaprio version of himself, and then populated his world with actors from some of his favorite movies like The Dark Knight, XMen, and Insurrection. (Ok, I know that last one is stretching it.) I also expected the movie to end in a much realer world where extraction is done on a more complicated device that required more than just a vague cable to be attached to your wrist.