I only use "have" because I personally think the "am" sounds like bipolar disorder is an identity. I am not a mental illness. I also believe that other illnesses where people sometimes use "am" should be switched to using the "have", like instead of saying "I am diabetic", I think "I have diabetes" is a better choice. To me, using "am" is almost like self stigmatization. Perhaps the people that prefer the "am" don't think of it that way, but others that hear it may be influenced to stigmatize it because of that usage.