The whole movie Inception is a dream within a dream.

I will accept this theory - only because it is impossible to foist scientific logic and procedure on the inherently surreal dream state as it relates to the real world. Therefore it is feasible that the film's primary (and thoroughly preposterous) plot imposition, i.e., the entire 'inception' process, has to be explained as a fanciful and ridiculous error within the dreaming subconscious of an immensely stupid and foolish individual. Then all the unreasonable nonsense that follows can be "accepted" as based on a false premise that is in no way connected to a rationally-derived universe.

The film simply cannot work if one believes that Titanic, Juno, Bane, 3rd Rock and their silly machines exist at the top-level in the waking world.

Let's see who takes the bait.

I think most of it was real, that's why we were able to see what happened with Yusuf, Arthur, and Eames on separate dream levels from Cobb.

Cobb had no awareness of what was going on with them, if they were projections they wouldn't even have existed because, remember, the world of the dream creates itself as the dreamer travels through it, nothing exists where he isn't. So they were all real dreamers on their own levels and the Inception mission was planned in the real world.

The only part that was a dream (which Cobb thought was real) was the end where he woke up and went home to his kids. We know this because the kids looked the same as they did in his dreams, down to their clothes and position crouched down on the grass.