Product review (2 stars): Snake oil....Performance is marginal and did not meet expectations.
Writing this review on 7/15 - I reside in the the central valley of California.  During the summer, the RH can take a significant dive.  For the last couple of weeks, the RH has been in the mid to low teens, not good for wood instruments.  Purchased this item with the goal of getting the RH up from 17% to the low 40's.  Change in the noted humidity went from 17 to 28% and it took 5 days.  Sponge is filled with water each night, guitar is stored in the case with the case lid latched closed and I do check on the RH each night when I practice.  Frankly, I have better results with the sponge in my cigar humidor, and its 1/4 the size of the sponge that came with this accessory.  I will add that I have used the hygrometer from my cigar humidor to see if the RH read out on the electronics might be off.  The numbers do track to each other.

Will give this a couple more weeks of testing, then if things do not improve, its time to look at returning it, and come up with an alternate solution.