A contributor on NBC's Today recently pointed out the fact that a humidifier will not only soothe irritated sinuses, but it will also lower your utility bill during the winter. The additional moisture in the air holds onto the heat; a humidifier increases the heat index, making 68 degrees feel more like 76 degrees.
I couldn't agree more. Having a humidifier will help with asthma too, because it keeps air passages soothed.