I have used some of the medical tech available, but this amazon stuff doesn't interest me.
What I use? Fitbit + Withings
Withings actually has some really good tech. Thermometer, scales, sleep mats, blood pressure cuff, and watches ...
The thing is they are reasonably accurate and good (their new scanwatch I can't wait to get), but you have to realize it is not a perfect measuring system.
You have to use the tools in away that you talk to your doctor and manage your health.
The fact that Withings is European based actually makes me feel "safer" then an American one which is kind of perverse ... but whatever?