It's a tough call. If someone is suffering, how can we refuse them the right to die by choice? We provide this for animals, calling it humane. For people, wouldn't it be more humane?
On the other hand, helping someone to die is like murder unless it is legalized. My biggest concern would be someone taking advantage of this kind of situation, talking someone into doing it (or coercing them) for personal reasons (inheriting money, etc.).
Taking a life is something that seems unnatural to me: I can't imagine taking another's life, even if a person asked me to do so. In some ways it might be the greatest gift you could give someone who is suffering as we cannot imagine. However, I would be haunted by it for the rest of my life. And I'd have the doubt: "What if...?" What if what, you might ask? I have no idea, and that's something makes it so frightening to me.