Like many others here, I also registered several years ago at a drive in my area. DD was a baby and she's almost 13 now, so that's how long it's been. Six or seven years ago, I was called for futher testing based on being a preliminary match for a female patient suffering from leukemia.
I was asked to drive to Nashville (2.5 hours away) to Vanderbilt University Hospital for further testing and counseling. It was an all-day event. I had an EKG, Chest x-ray and many different blood tests. I met with an anesthesiologist and a psychologist. I was willingly ready to donate.
A few weeks later, the transplant coordinator called with the bad news that the patient had taken a turn for the worse and was no longer eligible for a transplant. Apparently she was elderly (early 60's I think) and was right on the cut-off for transplant eligibility for age already. With her declining condition, the transplant was cancelled and she passed away.
I did receive an anonymous letter from her daughter several months after the fact. It was a very good feeling and I would do it again in a heartbeat.