I think questioning the science or the ending actually forces you to miss the more important points of the movie. The point of science fiction is, given that you ALLOW the science to be real, is the human psychology correct? If you question the science in a science fiction movie (at least, in a way that distracts from the point of the movie), you should have went to play miniature golf instead.
SPOILER ALERT
I think the point of the ending is largely missed. You're told the whole time that this spinning top tells you whether what you are watching is "real" or not. But, you are sitting in a movie theater, watching unreal science, with impossibly attractive people riding snow mobiles in Cillian Murphy's head. I'm pretty sure this is a comment on film-making itself. It's not that Leonardo DiCaprio is still in a dream. Because it's all a dream, because it's a movie, and you as the viewer are the one who is in a dream-like, escapist state sitting in a cold theater in a hot summer. The point is that the movie has created something wonderful, a dream-space, in which your imagination can run and play and you can be distracted from all that is so much less dreamy in the world.
In essence, the "trick ending" is "the kick" that sends you back into the real world, the thing that signals that your dream (the movie) is over, and that it is time to wake up and go to Costco to buy paper towels.
See, I think that's absolutely brilliant. I hate trick endings, and that's not what this really was. It's not Mulholland Drive. It's A Midsummer Night's Dream. I think worrying about whether Leonardo DiCaprio is still dreaming or not is the opposite of the point. And I think the latter can ruin the movie for you.