Just like abortion, I think it's about choice. Whether I would decide to do it myself in whatever circumstance is immaterial, because that circumstance will never -- can never -- be identical to ANYONE else's, and so cannot be compared -- but the choice needs to be there.
I do believe in having assisted suicide as a legal choice. I think there definitely need to be safeguards in place so that family members who are just eager to hustle someone off this mortal coil for their own selfish or greedy ends cannot do so. But, the circumstances where assisted suicide generally (and all generalizations have exceptions) comes up, are those
*where a person has a definitely terminal illness which will definitely cause them to have a period of lousy quality of life with at least some degree of burden on loved ones, prior to their totally predictable demise, and
*where there will also be a period of continued functionality and rationality between the diagnosis and that phase of incapacity.
That being the case, I would see it as entirely possible during that early phase for the patient to acknowledge what is coming, and make provision, by executing whatever legal documents might be deemed necessary, to say that once his quality of life reaches X predictable (based on his particular condition) level, he authorizes the ending of his life. We have living wills now. This is really not that different.
If he doesn't take that opportunity, while he is demonstrably capable of making that decision for himself, then unfortunate as it may be, I think he lives out his life, such as it is, without the possibility of assisted suicide, because at no other time is it possible for an informed decision to come from him. That's how people get protected against others making that decision for them, but I really do think the option of making the decision for oneself, and having it legally binding, needs to be available.
I hope, as a society, we catch up with Kevorkian soon. As has already been said, we have no difficulty with the concept of ending our pets' suffering, so why is it a problem to end that of a loved human, if it is clear that that is what he wants?