I really enjoyed the movie. I REALLY enjoyed it. I could take all day writing about the things I liked, but it would be hard to do that without giving anything away. Instead I'm just going to point out my one gripe, and that's the lack of character development and lack of chemistry between the characters.
I think that DiCaprio's character was fleshed out well enough, but I really wish Ellen Page's character was made a little more three dimensional, and same with Levitt's. I don't want to elaborate (for obvious reasons), but that's the biggest complaint I've got with it. The story got a LITTLE mechanical and I think it could have used a little more emotion and heart.
There may be a few plot-holes here and there, but the overall quality of the movie was so good that it's more than forgivable. When I walked out of the theater, people weren't analyzing it as a movie, they were talking about their OWN dreams and speculating on events that happened within the movie as if the characters were real people. During the end of the movie, people were literally leaning forward in their seats and I really loved seeing that level of immersion in the audience.
I'm really excited for Batman 3.