As for it being a heist movie, lots of the action sequences are boring. Only the free fall sequence is genuinely successful. The cliches about running from Cobol Corporation are so tiresome as to make people wonder whether the whole movie's a dream.
You know that did cross my mind during the scene where he's being chased by Cobol. Arbitrary villains... or a nightmare?
While I had elaborate theories in my head about maybe the whole thing being a dream (and I had a sneaking suspicion that Ariadne = Mol; I can just imagine Cobb being introduced to Mol as the star pupil in Nicholas Cage's class...); I'd generally discount that for the existence of opposite viewpoints in the film.
How did DiCaprio and Watanabe escape from limbo?
They don't. The film pretty much telegraphs this with two very obvious details at the end: That DiCaprio's children are unchanged; rising to greet him just as they may have done but did not in his memory (it's far too picture perfect to be anything other than a fantasy moment), and, of course, the spinning top. While I like to toy with the idea the whole movie is a dream; just cause, the cleaner explanation is that the movie is real and DiCaprio's escape and awakening at the very end is imaginary.