I absolutely want the ability to check myself out when the time comes. Thing is, I'm pretty sure I have that now -- so long as I can swallow a few bottles of pills. I understand that some folks who wish to die are unable to manage this small effort, but I agree with samclem about the dangers of having it be too easy or acceptable.
I remember my father once saying that he was looking into a reverse mortgage and my sister indignantly replying: "Don't do that -- you'll leave your children with no inheritance!" As much as I hate to say it, there are families out there who WOULD try to guilt Mom or Dad into shuffling off early.
If I have to choose between having some who want to die linger on, vs some who want to stay leaving prematurely, I'll vote for the former, as imperfect a choice as it is.
As to the Alzheimer's question, my DP works with Alzheimer's patients now and many of them seem to enjoy their lives, as Redduck suggests. They enjoy meals and eat well, they participate in activities with enthusiasm, they remember old songs, play bocci ball... Every day he tells one 90-year-old client, Josie, how beautiful she is, and every day she beams at him and tells him he's a doll, which he just loves. So not only is she still capable of joy, but she is still able to bring it to others.
I understand that this will change, but who is to say when we cross the threshold between a valuable life and one that is no longer worth living?