That Circadia device looks way too large to be comfortable. With some more engineering, it could have been made much smaller. Also, in the removal picture the battery seems to be bulging rather dangerously..
Battery maintenance and safety is a big concern. It's not easy to get at these devices once they're inside you. I know pacemakers use Lithium-Iodine battery chemistry, which is much safer and longer-lasting than Lithium-Ion.
Additionally, if it's just measuring pulse and heat, how much value does it provide over a smartwatch? My Moto 360 already takes regular heartbeat measurements and I can (crudely) map it next to my sleeping patterns measured from the accelerometer[0]. (Interestingly, there's a fairly high correlation between the two!)
I appreciate the vision of biohacking, but this device doesn't seem to provide much advantage over an external device, while introducing a whole bunch more inconvenience and risk.