More and more companies are coming up with mid-range smartphones and these smartphones are also popular and fits the pocket. Also IMO, for an average user there is not much difference for functionality between the top end smartphone and a cheaper version.
The S5 comes with a heart rate monitor app that fits into a framework of several fitness monitoring apps for monitoring steps, food intake, etc. That framework appears to be built to add apps later as well. It looks interesting, but only time will tell if it builds up or falls by the wayside.
I picked up one of these over the weekend. I am still adapting to it, but so far I have ignored the fingerprint scanner. I figured that it would not be secure, and recent stories seem to verify that. On the other hand, I have taken my pulse several times and am getting interested in tracking overall activity with it as well.