I have 4 air purifiers (bedroom, living room, hallway, and office).
The purifiers have an auto feature so if it detects any decrease in air quality it turns on. It is pretty noisy unless on low, but if you keep it on low it does do a good job of cleaning the air.
I have an off the charts severe dust mite allergy as well as other things and the purifiers have made a definite improvement. Another thing that really helps is this laundry additive
http://www.natlallergy.com/prod/1235/demite-laundry-additive-for-dust-mite-control.html
You add it to the wash and it kills mites. Just washing sheets in hot water doesn't work.