I donated 12 years ago to a stranger. It was an amazing experience, and I'd absolutely do it all over again.
Physically, yes, it was painful for a few days afterward. (I understand the procedure has changed and it hurts less these days.) I underwent a variety of tests to make sure I was healthy enough for surgery, and they drew my own blood ahead of time so that they'd have some if they needed it during/after the surgery.
But the physical experience was nothing compared to the emotional one.
Knowing that I may possibly have saved someone's life -- it's hard to put it into words, because however I try to describe it sounds trite. But it was a tangible way to make a difference and do some real good. I'm so glad I was able to donate.
Oh, and the staff at the hospital was *amazing*. They treated me like a queen and everyone -- from the check-in person to the post-op nurse -- was extremely grateful, even though none of us knew the recipient.
I matched with a 17-year-old girl in another country, and wherever she is (I suspect Japan, given the idioms in the letter and the type of gift she sent prior to surgery) does not allow donors and recipients to be identified -- ever. I never found out if she survived. I thought not knowing would be tough, but it wasn't.
Feel free to message me if you want more details.