I'm on the fence with mine. Hygrometer works every time, and it's tough to beat it for precision and accuracy. Where it loses is convenience.
I calibrated my refractometer on two points, distilled water and a 1.080 (I think? was a while ago) sugar water solution. Sadly neither of the scales my refractometer read in were correct, however using the "refractometer tools" selection on Beersmith corrects for this error, and it now matches my hygrometer.
Where I've really run into issues is that I can't always get a repeatable reading. My concern is that this device could be just as temperature sensitive as the hygrometer, but without any way of measuring that temperature. Maybe other on here can explain.
I use the refractometer during the mash, always take original and final gravity readings with a hygrometer.