To measure pulse / heart rate, I tried out the Pulse Sensor Amped.  

So far I can get results from either my fingertip or earlobe, with the fingertip being much more reliable - but moving my finger screws up the readings.  Other finger locations (e.g. back) are not reliable.

Having the sensor on a fingertip gets in the way of "doing things with your hands" so it's not suitable for a sensor that silently and invisibly watches you in the background.  

The earlobe location works but looks totally stupid so it isn't an option either.

I was really hoping to use my sternum/chest but readings don't work there.

So far things don't look great.  The sensor is expensive at $25, is not terribly reliable, is limited in the body locations on which it'll work, and generally the best I can say is that it mostly does a job that nothing else out there can do for a hobbyist price point.

The jury's still out but unless something changes it doesn't look like this sensor will make it into the final design (or if it does, it will be in an exhibitionary/experimental manner only.)