Product review (5 stars): I am pleased with this hygrometer
We own four of these little hygrometers.  I've found them to be accurate -- all are within 1-2% relative humidity of one another when placed next to each other (in general, hygrometers that are within 5% of each other are considered pretty accurate for low-cost, consumer-grade hygrometers).  I have them dispersed throughout the house, so I can verify the temperature and humidity in each room.

The price is pretty reasonable, too.  I like the look of these units, and their simplicity.  The only "option" you can select is whether the temperature is in F or in C.

The first one we bought two years ago, and the battery is still going strong.  I haven't checked the type of battery, though, to see what will be needed when replacing.

With this little tool, it's been easy maintaining the humidity in our house between 40-45% year round with the aid of three humidifiers.