Product review (4 stars): A convenient monitor for your instruments' health
I use this

Pros:
1. Appears to be accurate. I compared it to a more accurate hygrometer in the room.
2. The connection with a phone app is great because you can check at any time
3. The app shows you a history of humidity and temperature readings, includings highs, lows, and averages
4. You can set alerts when temperature or humidity stray outside defined boundaries

Cons:
1. The app touch targets are a bit strange. It took a while to figure out how to access the history because I was tapping the "wrong" part of the sensor icon
2. A bit costly, but ultimately I think it's worth it