I used to give blood and I still would if I could. I got to five donations before the blood bank people suggested that since it takes me so long to recover afterwards I should probably stop. If you've never donated, don't worry, that's not a normal reaction and probably won't happen to you. Most people are out of there and enjoying their free tea and biscuits in minutes.
I'm taking partial credit for my husband donating since I nagged him into going the first time. He's nearly up to 250 donations now because he gives plasma instead of whole blood so he can donate more often.
I think everyone who can do it should since blood and blood products are so badly needed, and you never know if you might need to be the recipient of them one day. It's such a small sacrifice to potentially save someone's life.