I purchased this unit a month ago and it has performed better than I expected.  I placed it on silent mode and with the indicators out I have to place my hand over the top to check for air flow since it is so quiet.  

When the local area has a sudden increase of airborne particles, the unit immediately responds increasing fan speed until the air is clear again then returns back to the silent unobtrusive unit that it is.

Great unit and definitely worth the money!