I don't, for three reasons:
1. When I have in the past, the water was hot and I felt like I was drinking piss. I prefer the piss I drink to be cold.
2. A couple years back I saw a scruffy, dirty looking older man approach a fountain. You know the part that the water shoots out of? Yeah, he put the whole thing in his mouth. I mean the way he had his mouth all over it, it looked like he should have bought it dinner and drinks first. It was just gross to watch. I know they clean them, but you don't know who used it and what they did to it five minutes earlier...(muhaha).
3. I usually have my own water, anyways.
Do you drink from the drinking fountains? Might seem like a "yeah, who doesn't?" question, but I know plenty of people who don't.