Fitness sensors and apps seem to be a common thing to put into phone/watch these days, but how useful are they actually? I do plenty of sport (jogging, cycling, playing hockey) and have done so over the last 20 years, but at no point in my life have I ever thought "Wow, I wish I knew what my pulse-rate was right-now!". It may be of use to professional athletes, but to the normal person who does exercise to keep fit or for fun, knowing your pulserate etc is pointless. To me, cramming these fitness sensors into phones and watches is a solution in search of a problem.
Having a portable device track your heart rhythm as well as your heart rate could be of great importance. My aunt suffered a massive right middle cerebral artery stroke, caused by a previously unrecognized atrial fibrillation. If she had some device that could have warned her that her heart rate was irregular, she could have avoided this crippling, life-altering event that has left her unable to walk normally or speak more than one or two words at a time.