Is't not that the procedure is painful to the recipient, it's afterwards that is so hard. It is just like a blood transfusion, with the new cells usually being given by IV. Not too bad.
Right afterwards, the person is considered immuno-deficient. Remember what the I & D stand for in AIDS. The form of Leukemia that my dad had, Chronic Myelogenous Leukemia, CML, basically causes the body to produce way too many white blood cells which effectively blocks production of other types of cells. I can't remember the numbers, but the white count of the average CML patient is off the charts. After the transplant, the white count is reduced to next to nothing. Since white blood cells help fight infections and stuff the body is susceptible to all sorts of things. Hospitals stays of a month or more after the transplant are the norm. During that time in the hospital, lots of blood transfusions are necessary to supplement red cell and platlet counts since those aren't being produced very well either. It'll take quite some time for the immune system to get back up to 100%, and during that time the patient is still susceptible to all sorts of wierd things that are often life threatening. A common cold can turn into some form of pnuemonia <sp> and kill rather quickly.
It's a dangerous procedure, and not something that is taken lightly. All sorts of factors have to be looked at. Since my dad was in his upper 40's he wasn't really a candidate for the transplant, but the chemotherapy (not normal chemotherapy, it was some sort of study that NIH was doing at the time that consisted of daily injections of Inteferon Alpha) was able to put the Leukemia into remission.
There's lots of websites out there that give info on transplants, I'm just recalling stuff from 6 years ago.
Here is a good one I just found.