Someone once described it as a visual poem, which it is. It's a masterpiece within the art of cinema, and not particularly within the art of narrative cinema. Basically, it's a long stream of amazing images and music. It's not as cerebral as many people say it is. You can watch it without thinking. It's an experience, or as the tagline stated ''the ultimate trip''.
Kubrick was a genius, perhaps the greatest genius in the history of cinema (When you think of how far ahead of everybody he was, you would imagine that he had one of those monoliths standing in his office to help him along with the evolution of cinema), because I think that in the future of this art-form it will become less and less dependable on stories and structure and more and more a flow of imagery, hinged on a theme or an idea. Maybe not in our lifetimes though.