I've done that twice. First time was kinda weird for me. I was a senior in high school and donated to see what the experience felt like. I managed to get through the whole thing fine until I stood up getting myself a drink before I started blacking out (about 5 minutes or so after donating blood). Someone asked if I was ok, and I somehow knew what they were asking and replied with "I think I'm passing out." They moved very fast to lay me down again and get a wet towel on me while keeping my legs up. I had to take it easy the rest of that day.
Second time: It was the day after 9-11 and I donated at 3 am in the morning... I missed the morning class that day but that's alright.
Both times, it didn't hurt much or at all... So all in all, it wasn't so bad I guess... But I still admit I absolutely hate needles with a passion. I don't even like getting my shots but I think my experience with donating blood has made me braver when I have to get my shots (hep b vaccine before college, etc. - that kinda shots).
