I have done so on many occasions(about 20), but it’s been a while since the last time. The last time, a vein collapsed and it was kinda painful.
They still call me, though. I’m O negative, the duct tape of blood, fits anyone in a pinch.
I used to do this via the Red Cross. Except for the loss of a couple hours every time I did it, it was easy and no more painful than giving blood.
One annoyance is that you can’t move your arms while the needles are in. If you get an itch on your nose you have to tough it out or get somebody to scratch for you. Otherwise, no problems.
Apparently, too, I have veins like subway tunnels so YMMV.