Product review (4 stars): nice tool; but...
bought 4 of these. one reads perfect. two more read 3% low on humidity (50% reads 47%). i returned the 4th one which read 4% high (50% reads 54%). so there was a difference of 7% humidity from the units I sampled. temperature was with 1 degree across the board. d'addario responded to my email saying that it was an acceptable variance. i disagree; i own a 10k guitar.
  obviously as long as you know what the variance is you can calculate the correct value so it is still a handy tool. works as well to monitor a "guitar room" as well as in a guitar case. if they read accurately i would give 5 stars.
  also, it works on bluetooth so you need to be near the device to monitor it so don't think you can read an instrument from a distance while traveling.