Product review (5 stars): Appears to be accurate
I tested this against a good scientific wet-dry bulb sling psychrometer, which I believe is as accurate as you can get. I confirmed that the temp readings of both devices agreed within 1 degree. It was impossible to compare the humidity readings of both to scientific accuracy because I was in a small room with the climate control running and thus there were both temp and humidity gradients happening in the room and there was no way to be sure the GSI device was "breathing exactly the same air" (so to speak) as the sling psychrometer (which much be physically spun around and creates it's own turbulence/air mixing). But I tested both several times and there was enough overlap of readings to make me think the GSI device is likely within spec per relative humidity readings. One could do a salt test to find out just how accurate it is but for my purposes it's good enough. If it does what it's supposed to do (it appears to) and lasts a long time (remains to be seen), what makes this item a big winner to me is the low price. I've wanted one for a long time but always held off because they're usually a lot more expensive.