I have had good doctors and I have had some bad doctors, but most of the doctors I have seen have been in between. In the mid 1990's when I was diagnosed with hypertension I bought a good automatic BP meter and have taken and recorded my BP regularly ever since. I also make notes when there are variations in either direction as to what MAY have been the cause, and try to make any needed changes in my lifestyle. I ALWAYS take my numbers to my checkups and most of the time the readings in the doctors office do not correlate well with the readings I get at home. I have even had it called "white coat hypertension" by more than one doctor. As a result of this over the years I have been able to reduce the prescribed medications, in agreement with my doctor, by well over half -- maybe more and my BP is within the normal range for me whenever I take it. And yes, I have checked the calibration of my meter.
Another issue I have had is the two lesser forms of skin cancer, many Basil cell cancers, and a few Squamous cell ones. Although I have a checkup by my dermatologist twice a year, most of the time I find something that I am suspicious of for him to examine. As recently as 2013 I had a very tiny growth very near my left eye that appeared suddenly in the late fall, shortly AFTER my exam. I was suspicious that it was a skin cancer and called and got another appointment for an exam. My dermatologist did a biopsy, which was positive for Squamous, and I was able to have Mohs surgery to have it removed before the end of the year. It was still small and the surgery was much less invasive than it would have been otherwise. If I had let it go until my next check up I would have had to have reconstructive plastic surgery in addition to the Mohs surgery.
While I am not a doctor, and never wanted to be one, I am very much in favor of any device that can let me monitor my own body and then find a doctor that will listen to me.