I stayed awake for three days once, and it was both one of the best and one of the worst experiences of my life.
I did it when I was still at school and had a number of big assignments due in all around the same time. The first day was completely normal as you'd expect. The second day I just felt incredibly tired the whole time and slightly zoned out. The third day was utterly bizarre. I would keep catching myself falling asleep at ridiculously impractical moments, like when walking down the stairs. Whenever I had to sit still I would experience the most vivid and realistic hallucinations, and accept them without question for a good 15-20 seconds or so.
I had to attend a piano concert on the third evening which was really strange, as it meant I had to sit still for about 2 hours while trying to stay awake. During the concert I repeatedly saw the pianist do perculiar things, such as opening up the piano and stroking it, as well as taking off his jumper and crawling underneath the stool he was sitting on - I have been assured that none of this actually happened!
Would love to do it again but expect I never will.