I actually have experience of the donation procedure. My brother received a kidney. The process is horrendous.
At 3am in the morning he was one of 3 people called to their local transport hospital for the potential kidney. They all had tests and the kidney was then transported speedily to the hospital of the person that was 'chosen' according to some formula that included how close they were to death without it, whether they had a cold that might impact on receiving and the fitest etc.
My brother was chosen. With every hour that passed without the kidney inside him the chances of success would dwindle. The kidney arrived but he was pipped for the theatre by another transplant of greater need, so he and his new kidney (the result of a car accident that evening) had to wait as the chances of success grew less and less.
He got his kidney and then spent a few weeks in hospital getting to know other people and their families and watch some of them go through the rejection process, some irretreivable.
His kidney worked at 80% (which is fantastic btw) and he was given a estimation of about 10 years until failure. He's had it 15 years and there are no signs yet.
And yet, I'm not on the register. However I have discussed it with my family and they know that the decision is down to them and that I give THEM consent to whatever they can handle. I don't want them to feel like they have let me down if they can't do it.