One of the best films of all time, not just in the Sci-Fi genre, but in general. Aesthetically it is excellent, which goes without saying for a Kubrick film. The scenes feel spacious and are very beautifully designed and carefully thought out; they easily draw you into the world of the film. As far as the script goes, it is just as brilliant - feels natural and tacit and gives the characters depth and authenticity. Everything is somehow simple, yet very deep and subtle with loads of attention to detail and originality. Its implied complexity mirrors that of real life in a way which is very compelling and I find that to be its best aspect. Also, many people have said it stretches out too much but for me its long runtime is a good quality. There certainly are films which are too long in a bad way but this is not the case here, since everything serves a purpose and connects to form a near perfect whole, which makes it an enjoyable experience. Its plenty of good qualities make it possible to watch many times over and give the viewer something new each time. However, I think that reading the book(s) on which it is based is very important in order to understand it thoroughly. I find this as a bit of downside, since it may be confusing to the part of the audience for which this is the first time encountering the story. Other than that it is a rare, truly marvelous piece of cinema.