Ok, I've got some perspective on this as the husband to one heart transplant patient and the son-in-law of another. Both passed away in the last year, unfortunately.
Both of my family members would have died within months of their transplant if they hadn't gotten them. My father-in-law got 12 additional years of life. They were relatively healthy years, too. He got to see his daughter married, graduate from college, see me commission, and get my pilot wings. My wife had problems with her first transplant. She was in and out of the hospital for some of the time. She had to have a second transplant and after that she had some steroid-induced myopathy and was in a wheel chair for awhile. Once she got out of the wheel chair, her main problem was migraines. She finally got those cleared up this summer and was very happy with few to no health problems for the last few months of her life. I told her once recently that she was very strong and I don't think I could do what she did. She told me she didn't think that was really the case, but that she didn't have a choice and the extra time was worth it, no matter what the negatives. She got an extra 6 years. For us that was enough to take us from 4 years of marriage to 10. They were good years. I know it's a huge cost for one small outcome, but to those who are part of it, it's very, very special.
So you see the downside exclusively. Remember the extra time and what that can mean to a family. My take away lesson is that transplants should be a definite last resort, but once you reach that last resort they're a wonderful thing.