My husband and I both have one in place, he has CF and I don't- but I don't think it really matters. I am a nurse in a hospital and I have seen too much turmoil not to have one for myself. We both have eachother as medical power of attorneys, then I have my mom as my secondary and he has his brother as his secondary (In case we are both in a car accident...or anything where we go together). A medical Power of Attorney gives the person that we give the "power" to, to make our decisions if we are unable to. You have to choose someone who you know is going to respect your final wishes.
It is difficult to find someone to do that somtimes, so with an advance directive you can put in writing and make it clear as to what you want to happen if for certain situations, like if you code during surgery versus if you code in a normal hospitilization situation (a "tune up") you can choose what you do want done and what you don't.
This is a very touchy situation, but I would rather have people know what I want in certain situations than fight over what might be my last days. That is just my position though.
Julie