I've been considering it for quite a while, recently reminded by this article. If it were a quick and painless process I'd almost certainly do so, but I'm not a big fan of the "spend months doing paperwork and hospital tests, then days or weeks in significant pain" part. I also have some philosophical concerns about anesthetics.
If anyone points me to a way to avoid doing most of the paperwork myself, and/or statistics that serious pain and complications are unlikely, that'll make YES more likely.
Clarifications:
I'm registered as a kidney donor upon death, and that method can count to resolve to YES.
If I die due to the surgery, that can still count as YES.
If there's a problem with the surgery and the kidney is not successfully implanted, that counts as NO.
If they kidney is implanted into another person but then fails shortly afterwards, that counts as YES.