I've done it loads, I've got a rare blood group (my mum always said I was special!) so I feel like I'm doing my bit.
It does make me a bit light headed after, but nothing too bad. It's a bit uncomfortable when they put the needle in, but it's more of an ache than anything else, then you just lie there with a bag attached to you for about 3-10 minutes (depending on how quickly you pump the pint out) and they pull the drip out, stick a plaster on you & leave you to have a lie-down for a bit. Then you can get up & have free squash/tea/coffee & biscuits, job done.