ok my father had a bone marrow transplant (although he sadly died due to complications with infections a few months afterwards).
i am all for people signing up these things who are serious about it, but please be aware that if you are called it is a big commitment! to donate bone marrow in the first place i believe it takes a couple of days and you have to travel to london, and then afterwards you may be called upon to donate to that patient again (this happened to my dads donor a couple of times, can't remember exactly what for). i would say to anyone only sign up to be on the register if you can guarantee you will be able to be there if needs be. i wouldn't sign up whilst having young children and a full-time job as it is not a commitment i can make. it also costs hundreds to sign people up so you have to be sure it's the right thing for you.
it is also not something to do if you want to be a hero. people who have bone marrow transplants do not go on to have the easiest lives and for the first few months/years they are constantly in and out of hospital fighting infections and complications which arise. many people (including my dad) die from these infections as their immune system is so low. not an easy thought.
despite the outcome of the fathers case, i am extremely grateful to the donor for giving him a chance. without it my father's cancer would have returned and it was likely he wouldn't have another 10 years. as it turned out, not having the transplant would have been the better option, but no one could have known that and in my dads case he had to take the chance on a transplant. it is a great thing to do if you have the time, but please only register if it is something you are serious about doing.