The first time I experienced this film I unfortunately didn't get to see the whole thing. I was maybe thirteen or fourteen and I started watching it at a friend's house with a bunch of people. Needless to say, pretty much everyone hated how slow it is, and it was turned off before we even got to the Jupiter Mission. As a result, I nicked the film from my friends house and watched it the next day. If I recall my little sister watched it with me as well, and to say that both our minds were blown, is a vast understatement. At least the both of us kind of had an idea of what to expect, as we had both grown up in a world were 2001 is regarded as a classic mind trip. The experience of seeing this must have been absolutely insane in 1968. Most likely the audience would have no idea what they were getting themselves into, and would have been unprepared for the super-civilization encounter at the end.
Definitely worth looking into the Kardashev scale for some added insight into the the end of the film. The idea of the human race encountering a super-civilization being the equivalent of an ant having a conversation with a human being, is a rather fascinating concept.