excellent

Rating: Null/10 (24 June 2003)

"2001" remains probably the best science fiction movie ever made. It is one of the few movies around that makes you think, and was made not with the intent of giving the audience any answers, but in asking them questions.
I would agree that reading Arthur C. Clarke's book version (which isn't just a novelization of the movie but was written parallel to the movie; for instance, in the movie the astronauts go to Jupiter but in the book they go to Saturn) is very useful in understanding what happens in this movie.
SPOILERS-------
The premise is very simple. Alien beings have decided to assist in and then monitor human evolution. The ape men at the beginning of the movie are vegaterians, going hungry not realizing that they can eat the animals what wander around their camp. They cannot defend themselves against predators or even against a rival clan that drives them away from their watering hole. The Obelisk appears and the Obelisk teaches the ape men how to start using tools. Armed with this knowledge, the ape men now become carnivores and start eating the abundant animals and then use their superior weaponry to drive off the rival clan and reclaim the pool of water----and note that one of the astronauts in the latter part of the movie is "Frank Poole."
The movie then cuts to the future and scientists have found another Obelisk deliberately buried on the Moon by the aliens. When sunlight hits the Obelisk, it sends out a signal towards Jupiter. The signal is a beacon to the aliens who now know that humanity has progressed enough to go to the Moon and dig up the Obelisk.
En route to the Moon the astronauts have to deal with their supercomputer, HAL 9000. HAL is supposedly incapable of error and yet makes a mistake which will jeopardize the mission. Unable to trust HAL to run the ship anymore, the astronauts have to turn off his higher functions. HAL discovers the plot though and murders all but one of the astronauts. The surviving astronaut is able to defeat HAL by turning off his higher functions----hitting him on the head if you will, just as the ape man in the beginning of the movie defeated his opponent by hitting him on the head.

The ship arrives at Jupiter to find another Obelisk orbiting the planet. The astronaut lands a craft on the Obelisk and is whisked away through some sort of wormhole or time warp until he is taken captive, if you will, by the aliens. He lives out the remainder of his life in this new place (or his life is accellerated by the aliens) and at his death the Obelisk appears again, this time to transform the astronaut into a super-being, the Starchild, who has nearly an infinite intellect and thus little need for a body, the final stage of human evolution.
This is certainly not a movie that anyone can immediately understand after the first viewing or even after multiple viewings. Perhaps others will disagree with me about what the movie's plot is or what the movie ultimately means. The dangers of technology permeate the movie as well---the ape man's primitive technology is used to destroy his enemy but in the future man's technology has developed a mind of its own and decides to destroy Man to save itself (the same premise used in "The Terminator" series).
Yes, the movie probably could've dispensed with a lot of the psychedelic stuff at the end (essentially the same sort of time warp/wormhole travel that is used in the movie "Contact" and probably to better and more understandable effect). I thought that the acting left something to be desired; none of the actors were well-known to American audiences and none of them went on to have notable careers--but I don't think the movie would've been all that much better had some superstar of the day, say Charlton Heston, played the astronaut's role.
In the end, it is a movie that demands that the viewer think about what is happening and think about it for a long time afterwards. There aren't many movies that did that then, or do that now.