Yesterday I donated blood. There was a blood drive at school, and I skipped lunch to donate. It's pretty incredible to make somewhat of a difference by just lying down for half an hour - however, if you're the type who feels faint for the rest of the day (like me), don't do it. They probably don't want your blood anyways, and there are other things you can do to help. And it will result in your lovely dormies doing things like carrying you home (this is very embarrassing; don't let it happen) if you do in fact donate.
Have you given blood before? Did you faint?