Personally, I think the individual should have the ability to choose whether they want to continue living, especially if they are afflicted with some debilitating ailment. There should be some access to safe, dignified medical alternatives instead of a bullet to the head or the end of a nylon stocking tied to the rafters. No one of us can know how some stranger feels about what they are facing, so to presume to force someone else to abide by our custom or preference is unacceptable.
As for those with mental deficiencies, why not allow them the same dignity of choice as "sane" people? If you want to look at it one way, if those with mental illness were permitted access to assisted suicide, might we not prevent at least some instances like the recent event in CT?
Government should have no hand in such decision-making, however. As mentioned by another, such ability would swiftly be abused.