I donate every 8 weeks. I usually donate on a Monday so I can be recovered enough by the weekend for my long workouts. I don't workout for about 2-3 days afterward because I'm so worn out and my workouts are crap for a few days.
I tend to have trouble with dizziness after donating, so I put ice packs on the front and back of my neck during the blood-letting and drink a Gatorade during and I'm fine afterward.
It will affect everyone differently. You won't know until you try!

I feel like it's my duty to donate because I'm healthy and able, and hopefully, if I should ever need blood, there will be enough healthy and able people who were just as willing to give to save my life.