It's quite simple, really: no greater piece of pure cinema has ever been created. And despite being a product of the Space Race 60s, it holds up remarkably well. If anything, the Kubrick/Clarke vision of 1968 continues to be ahead of its time.
The ambition of the picture is no less monumental than to tell the story of mankind's place in the universe. It is well worth seeing, then, for its reach alone, even if it doesn't succeed for you the way it did for me.
I've seen 2001 many, many times over the decades since its release, and it's been interesting to see the evolution of the audience reaction to it. During the cynical 70s, awe and wonderment gave way to nervous laughter at certain moments that weren't funny at all. As the paranoid fantasies of the previous era were seemingly becoming very real in our daily lives, HAL came to embody them: an authority figure run amok. Lately he's become the perfect symbol of Big Government for the right wing crowd.
Ah, the symbolism... It was much debated back then in '68, as if there had to be one true meaning to the picture. But of course there isn't. The fact that there could be debate only demonstrates how much Kubrick left to the imagination. It wasn't as if he put some sort of Socratic dialogue on the screen, leading you to a particular point, or point of view. His imagery is wide open, bold and often inscrutable (that dread, staring, engulfing monolith). The work dares you to find a single interpretation for it.
One final note: you must see 2001 on the big screen, with the best possible sound system. It is one movie I will never watch on TV. A few films just do not scale down to the small screen at all (Apocalypse Now also comes to mind), and 2001 heads that list.