A few months ago I purchased the iThlete HRV finger sensor. I had big hopes to take some HRV measurements for not only myself but a few patients and some athletes that I train. It was 75 dollars and I believe the app for your phone was 10 bucks. I liked the idea of not having to strap a heart rate monitor every morning and also having to hook up the chest strap to other people.
I have a Samsung Note 3. My first reading was -7. Negative. I asked someone that knows a lot about HRV and he had never even heard of that. The next few readings were in mid fifties, then all of a sudden I'm at 99 for a readings. Let me backtrack...I get these readings when I get a reading. Most of the time I've never been able to get one and I'm continually told there was an error.
As you can imagine this is pretty frustrating. So much so that after 3-5 tries, I just quit as I can feel my frustration rising. So honestly, I wouldn't recommend it. But, my one caveat is that perhaps it's my phone and people with iPhones would be fine.
Either way, I can't recommend it from my own experience.