Thanks for the replies, everyone.
To clarify: my main motivation for buying an air purifier is for dust. For some reason, our home accumulates a lot of dust in a relatively short period of time, even during the winter season like now, when we usually keep all of our windows closed. I'm hoping an air purifier will help out with that.
We found an air purifier helped with dust. We also have a problem with dust. We live in sandy New Mexico and two large dogs coming in and out. Plus the house is a recent build energy efficient, so once some dust gets in the house it stays. We only use a MERV 7 HVAC filter to avoid overtaxing the HVAC system. After researching air filters I couldn't decide, so I bought what Costco had, the Winix brand. We now would recommend that brand. They are quiet and clearly do the job we needed them to do. And since we have had them our HVAC filter has less dirt and dog hair in it.
Our Master Bedroom doesn't ventilate well and DW was having snoring and dust issues, so we purchased the "Winix C555 Air Cleaner with PlasmaWave Technology" from Costco. It covers around 370 sq. ft. We are totally impressed. We can tell walking into the room that the air was cleaner or fresher than the rest of the house.
More of interest to you will be the second unit we purchased at Costco, the 'Winix Tower XQ Air Purifier', for $570. It says it is tested at 698 sq ft, but we have a 1000 sq ft living room/kitchen area we put it in. It does the job, even though it is at the maximum size for the unit. I did research it and there were other users with this unit placed in areas up to 1000 sq. ft. We are happy with it. Because the air circulation is good in that area, we didn't notice the dramatic difference we saw in the Master Bedroom, but we certainly can tell the difference in the air quality, especially with the kitchen in the area.
We purchased a third Winix C555 unit for the office area I work in most the day. It will often have a person and two 60 lb dogs in it for several hours every day. It helped there also.
So the air filters we purchased helped reduce the dust in the house. Before we had them, we would see the dust floating in the the streams of sunlight coming in the windows. After we started using the air filters, we no longer see dust in the sunlight streams. Plus our sinuses tell us the dust is gone.