Product review (5 stars): This is one of D'Addario's best gadgets.
This handy little device is easy to use and seems very accurate.  The bluetooth feature is a much needed addition for monitoring temperature and humidity sensitive enclosed environments like instrument cases, gun safes, and so on.  You can download the necessary app for free.  It is currently limited to monitoring 7 sensors at a time.  This sensor uses easy to replace and inexpensive CR2032 batteries.  A winner!