I donated blood for the first time when my high school had a blood drive. I ended up with an overall good experience from that, but I also managed to get a 3 inch in diameter nasty looking bruise as a souvenir (they had a hard time finding the vein). Ironically, that was the time that donating blood caused me the fewest problems and the least amount of pain (go figure).
Since then, I've donated a couple of times, but I didn't always have a lot of opportunities as a college student, and I was disqualified for at least a couple of years (nose piercing, then travel to Kenya). The truly unfortunate thing is that my last experience hurt a lot, and left me feeling very lightheaded and nauseated. I regret to say, I haven't been back since.
I really ought to suck it up, though, and just ask for someone experienced. I'll have to make sure I'm eligible, since I've recently done some other international traveling (and I'm not sure if it disqualifies me or not).
Also, when it comes to blood types, I've always been really interested in how they're distributed among the population. I think 70% of Americans have either O+ or A+ (it's about 35% for each of those). B+ and AB+ are a lot less common, but any of the Rh- types are really uncommon (probably because Rh- is recessive to Rh+). It's just amazing to me that O+ is as common as it is, since O is recessive to both A and B types.
And, to share a funny story, the first time I donated blood, they mailed me a little card with my blood type and some other stuff on it. They listed "unknown" for my blood type, which I thought was really very odd. I'm hoping that means they just weren't able to use it (although if they didn't even type it, that makes me think it went bad, and that's rather a waste). My actual blood type is A+ (which is yet another reason I ought to be donating more, since they try to match the blood types of donor and recipient exactly).