Great little smart watch for the price. Heart rate and oxygen monitoring appears to correspond quite nicely with other devices. Sleep monitor seems good too and deep sleep appears to correspond well to how I feel I sleep. E.g. if I get woken by my alarm and am woken with a start then it says I was sleeping deeply and when I get woken and am half awake, it k is I was sleeping lightly. All very clever.
Notifications and music controls are actually pretty helpful too, as is the weather report
Battery lasts from 5-7 days depending on how much you use the features
All in all, for the £13 I paid for this it's an absolute steal.
Only downside is that it's hard to get the supplied strap as tight as I'd want because it's slippy when putting on.