Product review (5 stars): Fantastic innovation; fantastic customer service
I bought two - one for each guitar.  Using two at the same time poses a bit of a convenience and/or storage issue. According to the instructions, you have to keep each device ten feet apart from each other. I had to find another place to keep one of my guitars when in the case. The way my "studio" room is laid out, it wasn't a problem.  I had a problem setting up the devices, but it was like a glitch that I was able to correct. It was as if the app didn't recognize a second device's own readings, but, "poof" the device seemed to correct itself.  A finicky aspect of Bluetooth?  Trusting the readings are correct, I love this device.  I receive a notification on my iPad when a guitar case gets too humid!  I then place a dehumidifier in the case and I am able to track the level of humidity without opening the case.  You can also reset the factory defaults. For example, it is set to alert when humidity reaches 60%.  I do not want the humidity to exceed 55% and I was able to adjust the notification alert.  (Instructions state it sends notifications after three days of improper humidity levels, but I receive a notice immediately.).  My only issue is that I am repeatedly receiving notifications that the battery is low for both devices. I open the app and see it is not low, as there is a battery life indicator. Then, the device realizes this, too, and turns off the warning.  Maybe, they have to upgrade the app?  Bugs are common in new apps/software, so I anticipate future app upgrades.  That being said, I highly recommend this device.  Even with the battery notification glitch, it still works.

Update: one sensor will no longer will no longer connect. I've reset the battery multilple times to no avail. I suspect my other sensor will stop connecting. Both sensors began to lose connectivity, like a flickering light, which delayed connection. Now one sensor will no longer connect - like a burnt out light bulb.

SECOND UPDATE: First, I want to mention that I overlooked the customer support offered when I wrote my initial update. However, thru this review, the company has contacted me and has proved to have outstanding customer service.  There were in fact, bugs in the app, which have been fixed. Actually, my original review cites a glitch with the battery indicator.  That wasn't a glitch.  One bug in the app drained the battery.  The company has been on top of problems with the app and has been updating the app as bugs appear.  They were able to pin point the problem I was having and they are going to send me a new battery, too.  Customer support is very friendly and committed to perfecting this great innovation.  It is typical for any new software or app to have problems (bugs) when it is released. D'Adarrio's commitment to quickly fixing bugs by updating the app and working with customers is commendable.  I highly recommend the Humiditrak and D'Addario.