Make no mistake. This is not an ambiguous film left open to interpretation. Christopher Nolan has made a living crafting intelligent films that thematically explore the differences between our perception of the world around us and the world that really is. Inception is no exception.
After having seen the film for a third time and poured over the countless interpretations and analysis online, one can come to this startling conclusion when when it comes to the issue of the top and whether or not it stops:
It doesn't really matter.
Whether you think Mol and Cobb's team are simply projections of his own subconscious (it was all a dream) or you felt that the top was going to topple and Cobb had returned to reality, the structure of the story remains the same. The meaning stays consistent. Cobb changed his resolve, and in doing so found both success and relief from his own personal demons. The success came with Fischer's notion that he be his own man and break up his father's empire. Relief came from reuniting with his children and seeing their faces once again.
How do we know Cobb has changed?
In the beginning of the story he spins the top and puts a gun up to his head, showing that he clearly has some issues with reality and his perception of it. At the end, though, Cobb spins the top and walks away. This is why it doesn't matter whether or not it was all a dream--the important part is that he refuses to be driven by that issue anymore. He has adopted a new way of appreciating his world and has triumphed in the process.
Because Cobb represents the emotional heart of the story and the audience's eyes into the problems at hand, it only follows that his blissful blindness at the end--whether real or imagined--becomes a shared experience with those sitting in the theater. In believing that we can figure out whether or not that top stops (and YouTube clips like this don't help), we become as oblivious to the truth as Cobb himself has willingly become.
And for that alone, this film ranks as pure brilliance.