Check this out, I gave blood for the first time. There was a bloodmobile near the local library and they've talked me into it. They took 500 ml. That sounds like a lot. But I felt perfectly fine. Did not feel dizzy or anything like that. However, they told me to skip gym for today. So, I did.
So overall, it was pretty interesting experience. We you get a chance, you should definitely do it. I am sure it would help some really sick person out there.