I've probably never really gone much longer than 22 hours without any sleep... Although I've gone longer with only very small amounts of sleep.
Probably the longest single stretch was from about 7:30 one morning to 7AM the next while I did some lab report, although I'm not quite sure if I dozed off for 20 minutes around 5AM. Anyway, went to bed at 7AM, got up at 8AM, went to all my lectures, feeling a massive buzz all day, had an hours nap before dinner, and other than that was normal all day.
I find that after initial tiredness has worn off I get what feels like a constant adrenaline rush and feel very awake, alert, and motivated as long as I keep myself occupied. My hands shake a little, but that seems to be it really.