I had the wonderful opportunity to donate this way earlier this month. It was totally painless (modulo a couple of needle pricks) and just required some time hooked up to the machine (~6 hours) and a course of drugs beforehand to boost stem cell production for a few days.
For the benefit it can bring someone it is a total no-brainer decision, and I'd do it again without a second's hesitation if necessary.
Thanks for adding it, people need to know this can be as simple as hooking up an I.V.