I've been on the donor list for years.
In undergrad, I gave lots and lots of platelets to the Red Cross. At one time, they had me in every 30 days for two pints. There was someone going through leukemia treatments I was a good match for. I haven't given for awhile, but I should get back into it.
Patelet typing is pretty close to bone marrow matching so they'll put you on the list if you want.
I got called up once in 1999. I went into the hospital for some blood draws to see if I was close enough. Unfortunately, I wasn't a good enough match.
Though I was mentally prepared to go through with it. I probably would have chosen general anaesthetic (the big, square needle isn't something I'd want to see) and spent the night in the hospital. It'd be a pain in the ass (literally), but recovery is pretty quick.
I do wish it wasn't quite so anonymous. I think that the donors and recipients can contact each other, but I'd want updates on progress and recovery. If you went through all that to donate, you'd want to know if it worked.
Anyhow, good on you for signing up. Everyone should. If you've ever wanted to save a life, this is a good way to help. Also, platelet donations are important. Blood donations sometimes sit around for weeks or months before they're used. Platelets usually get used within 24 hours and the people who get them really need them.