Pros: fine mist control, timer, and hygrometer.
Cons: moldy smell when in operation -- originating from the electronics/fan housing.
As soon as you plug in this unit, that water in the base is pre-warmed. This water is warm 24 hours a day regardless of whether or not the unit is turned on. Warm water grows MORE germs than either cold or hot. I feel that this pre-heating is a GIMMICK. With the water slightly pre-heated, the mist is finer than on other ultrasonic units. This allows the sales people to brag about how air-o-swiss has "finer mist". When you press the heater button, you do get warmer mist, but there is no way to disable the heater altogether.
Now I have an humidifier that smells moldy that I paid $55.00 for (I bought is used) STAY AWAY from Air-O-Swiss. I'm probably going to end up putting this in the trash.