Material quality seems not satisfying for me.
Sleep measure not accurate

It's not perfect, but it's definitely the most cost effective way to get Heart Rate Variability and other readings. I emailed the seller and got some info you might want that isn't necessarily included in the packaging:

- HRV readings are taken every 10 minutes from 12am-7am every day. It has a 0-100 scale for your HRV/stress, "abnormal, mild, normal" levels, as well as milliseconds for each reading. They responded that the ms is time between beats, but at 47ms I'm pretty sure I'm not at 1276 heart beats per minute, so it must be change in time between beats. Just use the 0-100 scale.
- I asked if you can use a fast charger and they said it only supports a regular charger (1.2 amps not 2.4 amps)
- It automatically detects your sleep times (so far this hasn't been accurate as I don't move around much for the hours before I sleep).
- I thought the o2 reading wasn't real, as it reads 98% even off my wrist, but then I took a reading as I held my breath and it slowly went down to 94%, so it works, it just detects a lack of oxygen.
- The 4 "watch face" designs aren't great. It looks like a kid watch if you show all the data, but at least you can choose between analog clock, digital clock, or 2 ways of showing the data.

Overall, it could use some more information and I wish it connected to other apps like Elite HRV, but this is better than a fitbit (why haven't they gotten HRV yet?) for a much cheaper price, so it deserves 5 stars.

Great watch, accurate heart rate and oxygen levels, it even alerts you if your oxygen level gets low at night. It tracks your rates throughout the day and night. I love that it charges from a standard USB port without any cable - just look inside the band!

REVISED:
Awesome "fit-bit". The only device i have seen that actually will alert you with a vibration if your O2 sat levels drop at night. Many features - take a little to figure out the app , but once learned - it seems to be awesome.
After 2 nights/days of use, here is the bad:. The charts provided with the cell app (have to have) are so small that it is impossible to read without holding a few inches away. Also the display is barely bright enough to be read in daylight. If you want to even know the time, you better have some shade available. The sleep indicator times are totally inaccurate. The concept is GREAT, but flaws and defects exist that need to be addressed.....

an excellent watch - easy to use and charge- accurate measurements