Product review (4 stars): Effective at reading humidity and temperature, (but not providing it).
I didn't read the description carefully and thought that this humidity reader would also provide the humidity to the guitar.  It doesn't.  It will tell you the humidity and temperature by inserting into the sound hole, but I had to buy another humidifier.  Maybe having the reader and the water in the same item wouldn't work.  I't is run by a battery; I'm not sure how long the battery will last yet.
Over several weeks, it seems to be very effective at reading the humidity, although I have no other instrument to verify its accuracy.  I can rest easy knowing my PRS Angelus Custom is safe from the dry air gremlins.  If you found this review useful to you, please mark it as helpful below.