Artistically it's a masterpiece without no doubt, it's incredibly inventive like most of the Pixar movies and deeply poetic like just their greatest ones.
It manages to use the animation to create and rationalize images, worlds, characters and concepts so complicated and imaginative that usually can only find their form inside minds.
But at the same time narratively it feels rushed and this makes it hard to be involved, moved and you find yourself detached; so many aspects about the main character, the dimensions he cross and about the philosophy behind them and the main story needed more attention, a more elaborated development, especially in the first half where the movie don't find the right rhythm running and running without a breath, a beautiful, essential breath.
Maybe it just needs to be watched a second time or even more to be fully appreciated.
Still it really is a good movie, even a masterpiece under some aspects as already mantioned, but it could have been a masterpiece under every aspect if it lasted longer.
It had so much to say and too little time to do it.