I've done it plenty, although not in years. I was on the plasma softball team and I ended up dating one of the girls there.
Not since college, though.
What's it like? No big deal to me. It was just kind of boring. Sometimes people pass out or get sick. We used to bet on who would get sick. We used to race too. Drink lots of water before you go and it will go a little faster.
The first time sucks because you have to watch some stupid video and get a physical and you're there forever.
They forgot to hook me up to the machine before they took the clamp off the line once, and my blood started spraying everywhere. They tried to clean me up, but I was covered. When I left, I asked the girl at the counter if this happened every time, with a really horrified look on my face. Two or three newbies got up and walked out. It was great. I miss college.