I've seen "Inception" twice, might go a 3rd time, can't wait to own the DVD. If you're willing to pay attention and think a little more than with most summer movies, you'll be richly rewarded by the gorgeous, inventive film. It's "The Matrix" crossed with "Being John Malkovitch".
Second viewing seemed to confirm my personal theory about what's going on in the movie. * SPOILER FOLLOWS *
Mol is running an inception on Cobb. She's planting the idea his world is not real, hoping to wake him up from limbo. He should have jumped from the building with her. The top will keep spinning at the end, but the fact he even spun it means the doubt is starting to take hold. A good sign her inception will ultimately succeed.