Product review (2 stars): Cool idea, but doubtful accuracy
Cool idea, but I have significant doubts about its accuracy. I bought this at the same time I bought a cigar box digital hygrometer (the  Caliber IV), and have been comparing them to the desk Acurite room hygrometer I've had for several years. The Caliber IV ($24 on Amazon) is cheaper than the Humiditrak, and is advertised to be calibrated at the factory. The higher price of the Humiditrak would be fully justified if it was accurate. While I found the iPhone app and the Bluetooth link to the Humiditrak to be easy to use and reliable, I don't feel that the Humiditrak itself is accurate. Before putting the Caliber IV and the Humiditrak in the cases of my two guitars, I put them beside the Acurite for several hours to compare them. I found the Caliber IV and the Acurite to be within 1 percentage point of each other, but the Humiditrak was almost 9 percentage points higher. Also, when I replenished the simple soundhole (ex-Planet Waves) plastic sponge holder sponge I use in each guitar case, the Cyber IV showed an increase in guitar case humidity percentage, as would be expected, while the Humiditrak remained unchanged.

I'm no scientist, and my observations are unscientific and anecdotal, but the two data points of (1) much higher reading than Cyber IV and Acurite for room humidity before using it in a guitar case; and (2) lack of change when guitar case humidity sponges were replenished leaves me with a lack of confidence that the Humiditrak is accurate. In the nearterm, I'll mentally subtract 9 points from what it's telling me. Eventually, however, I will probably replace the Humiditrak with a Cyber IV hygrometer if I'm unable to find a way to scientifically test and adjust the Humiditrak's measurements.