Product review (3 stars): Useful within limits
Useful if you understand the limitations

Pro:
Can be used to alert you to drops in humidity in your guitar case or guitar storage cabinet.
Also tracks temp and shocks.
Sends alerts to your phone.
You get a nice graph of humidity and temp changes, which tells you about your guitar's environment. I used it to track changes in a new humidification system and also to track if the way I was humidifying my guitar room was effective  (it's my home office, has a door and small enough to humidify.)

Con
Phone has to be near unit to pick up data. Data is sent via Bluetooth to the server but the phone picks it up later.
Was a bit fussy to set up. I had to repeat it a few times to get it to properly start sending data. I discovered that I had to turn my Bluetooth off and then on again and relink the Humiditrak each time (near the guitar case) to get it to recognize the device and start downloading data it had collected while offline.
Keeping the phone near it for continual humidity and temp info drains the battery and is not practical.
It's not cheap for what it is.

Summary:
Within limitations, this is a useful device to track temp and humidity. I have not tried the shock recorder. That is said to drain the battery further but it could be useful if you are traveling with your guitar and want to track how stable your transport really is.

SInce your phone needs to be near it to pick up data, you might as well put a cheap hygrometer and thermometer in the case and look at it.