I'm another 'yes'.
I'm not sure that I approve of someone being able to say too far in advance that they don't want to be resuscitated, though. I'd be fine with it being part of a pre-operative discussion with a surgeon, or at some other time when it might be a likely scenario, but I wouldn't be happy with it happening down the line, in circumstances that the patient might not have considered.
If the patient is compos mentis, and is suffering from an incurable disease, then I think it is cruel to insist that they stay alive, and even more so to take steps to ensure that they do, and would absolutely support a law that allowed for euthanised death.
In response to Janeainsworth's question, yes, I think I would be ok with administering the fatal dose, although of course I wouldn't want to do it, and am pretty sure that it would never be allowed. A doctor or nurse with the proper training (and who was in favour of the process) would be a far more sensible person to do it quickly and painlessly, with the full consent of the patient.
I understand (but don't share) religious objections, but don't really understand why people are uneasy about it because of a lack of faith in human nature. Any changes in the law would have to insist on tight regulation and procedures so it would be impossible for someone to bump off a rich but ailing relative in the hope of an inheritance or such.