Not very – according to a review recently undertaken by CNET.

Senior Editor Sharon Profis collaborated with Cardiologist Dr Jon Zaroff to compare some popular health tracker devices to medical ECG equipment at rest and during treadmill exercise at quite high heart rates.

Dr Zaroff explains that whilst heart rate monitor chest strap devices are essentially a single lead ECG in a sweat proof packaging, and with very good accuracy, wrist worn optical pulse measurement sensors depend on patterns of light absorbed & reflected from blood vessels beneath the skin surface.  These light patterns are sensitive to skin colour, blood vessel density and any relative motion between the blood vessels & the device itself.

The table below is taken from the article and shows significant errors for all the devices, but most especially the wrist worn ones at typical exercise intensities.

Device Error (%) at 80-90 BPM Error (%) at 160-170 BPM
Garmin VivoFit 10.7 0
Withings Pulse O2 5.3 57.1
Basis Carbon Steel 10.2 57.9
Samsung Gear Fit 4.2 Unable to read
Samsung Galaxy S5 3.1 0.2

As the article also points out though, measurements taken by passing light through the finger tip are inherently more accurate, and this is the principle we chose for the ithlete finger sensor.  To put the needed accuracy for HRV into perspective, the ithlete app measures each heartbeat interval to a precision of 1-2 milliseconds which equates to <0.2% error at a resting heart rate of 60 bpm. When validated by the University of Sydney, the average heart rate derived from the ithlete finger sensor was essentially identical to that from the reference ECG.  The dedicated pulse sensor of the Samsung Galaxy S5 also measures pulse at the fingertip and acquitted itself quite well.

Although many of these devices carry disclaimers about accuracy, they are not cheap toys and I think consumers have the right to expect reasonably informative (ie accurate) heart rate data out of them both at rest and during exercise. Needless to say, none is close enough yet to use for HRV!