Product review (4 stars): Wonderful, but needs improvement
Been using the humiditrak for a few weeks now. I attached the sensor INSIDE the actual guitar to the side using a Command Picture Hanging strip which is similar to velcro. In this way, I know the exact conditions of the guitar in and out of the case. Also bear in mind that the humidity levels are different in various parts of any given guitar case and may not reflect the actual humidity inside the guitar itself, which is where it matters most.

What I like:
-Keeps record even when out of range and updates when you get reconnected. So far this is the only solution that allows you to keep record hour by hour. Other in case hygrometers will only tell you the current conditions when you open and see it. This has allowed me to make several observations such as how effectively my in case humidifier works, how quickly the humidity inside the guitar drops when I take it out for practice, if my guitar changes temperature when I walk outside to my car on a cold winter day, what the conditions were while the guitar was checked in a plane, etc. These things you simply can't do with other hygrometers.
-Having the hygrometer inside has allowed me to make observations of what humidity and temperature my guitar sounds at its best and worst.
-I'm able to track the humidity/temperature while out of the house by connecting another device. I have a Nexus 7 Android tablet as well as my Android phone (Oneplus 3T) connected to the humiditrak. Interestingly I didn't have to pair the sensor with the second device, yet they both have the ability to connect directly to the sensor via Bluetooth. When I am out of the house, the Nexus 7 tracks the humidity and uploads the data to the cloud via wifi. As long as my phone has wifi, I also see the updates on my phone wherever I am.

What needs improvement:
-The sensor seems to only gather data in EXACT 30 minute intervals. In between those 30 minutes, I cannot force an update or reading it seems. I understand that the 30 minute interval is probably to save battery, but I wish I could force an update through the app if desired... a lot of conditions can change within a 30 minute period.
-I wish the app could allow you to calibrate the humidity reading then restore to default if desired. I compared the humiditrak with 2 other digital hygrometers and the humiditrak read 5-7% higher than the other two. I suspect the humiditrak is pretty accurate since I put two humidipaks inside my guitar when I went on vacation for a week and the reading stayed at a solid 50-51% the entire time. Still, I think there should be a calibration feature and should be easy to implement in the app.
-Since I started using the humiditrak with a second device, one device or the other or both randomly loses Bluetooth connection inexplicably and will not reconnect. Sometimes it connects instantly, sometimes it doesn't even after several hours even though I am only a few feet away. This lack of reliability is concerning.
-I'd like the ability to export data from the app to a tablet or computer for future reference.

If my complaints are addressed, I'd easily give this device a 5 out of 5. I believe the whole concept of the humiditrak is by far the best idea in instrument care.
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Update March 30, 2017 - I've discovered that if my device isn't connecting to the sensor, I just need to turn the Bluetooth OFF then ON on my device and they suddenly discover each other.