I don't know whether I've done exactly 16381 steps today, and it is pretty easy to fool if you want to, e.g. the floors climbed is certainly broken if you live somewhere hilly; it thinks mowing my parents' lawn is aerobics; I regard the heart rate as uncalibrated and purely wrist based activities (like beating eggs for a meringue) get counted as steps. But, it reminds me when I've been lazy, sitting in one place for ages, how often I was disturbed in the night (which I forget but am still tired) and stress shows up in the relative resting heart rate. So, on balance I like it as a way of reflecting on my day to day care of myself. I doubt my doctor will ever use it as the sole basis of anything.
When I next get a new phone I'll get one that talks to it, so it can buzz when I get a call. Mrs Dan will like this, as I might actually notice it.