Product review (2 stars): Maybe I got a dud...
I've had this for a couple of weeks and compared to the 3 other digital hygrometers I am using this one reads from 4% to 10% higher.  Sometimes these things read funny for a while and then recalibrate - no such luck on this one.  The other hygrometers I have read exactly the same temperature and humidity as one another when placed side by side, but this one is not only inaccurate it's inconsistent in its inaccuracy making it pretty much useless.  I like the Accu-rite digital hygrometer (for around $10), but it's a little larger (almost the size of a pack of cigarettes); and I wanted a smaller one to stick in a guitar case with double sided tape.  There are other small humidity meters that work fine - I have one by "Neewer" that works fine (I think I paid around $8), but for the price, I prefer the Accurate because it also gives you the "highs and lows" for both temperature and humidity so you know what range you're working with.  The Neewer looks identical to this one, so if you need something small, maybe try that brand.  The annoying thing is: unless you had other hygrometers, you might not know the one you had was a dud... I think when you buy a gauge you should be able to assume that it's reasonably accurate...