I'm afraid I'm not even sitting on the fence on this one, I'm on the other side and leaving a dust cloud as I peel rubber in a retreat. When I was younger, I thought I'd try anything twice - the first time for adventure, the second to confirm if it was an isolated incident or how that normally is.
The first time I donated blood, after stabbing me three times in the left arm, the nurse stabbed me in the right arm and said "Oh, I can't get the needle in. It's on top of the vein. Hold still and I'll go get somebody else." That hurt like... well, later, I'd find out it was about like a shoulder right after you reset the dislocated joint. But I was young and didn't know that yet, nor had a vocabulary foul enough to properly express it yet. I got out of there before the juice kicked in, because I fainted about two hundred feet away.
The second time, I warned them that the most luscious-looking spots for the vampires, on both arms, have valves in that exact spot. The nurse, even when warned, nodded, smiled, and stuck me through the valve. And then wiggled the needle around when the blood didn't flow, while I found myself mentally cataloging her ancestry, present worth, and future prospects over the screaming bone-deep pain radiating up my arm. Then she decided it wasn't working, tossed the half-full bag, and took a full pint out of the other arm. Through the other valve. The juice that time was guava, and my body wasn't having any of this placating: it rejected everything out of every orifice about five minutes after I stood up. I still avoid guava juice, though not as strenuously as I avoid blood drives.
Which is why I signed up for organ donation when I got a driver's license, and signed up today for bone marrow donation. You can't pay me enough to get me to donate blood a third time, so I'll focus on the ones I can do, and leave the rest to you.