BlueAir line 501
The BlueAir 501 is the mid-sized entry in the BlueAir line (the 401 being the smaller, and the new 601 being the larger). After doing much research on air purifiers, needed for a family member with severe allergies to dust (mites), mold, pollen, and animal dander, I decided that the BlueAir was the best choice without going for an "industrial" solution (i.e., creating a negative pressure clean room, which you can with the IQ air purifiers).
I have not been disappointed. As promised, it is quiet and extremely effective. It has 4 settings, and is completely ignorable at the first two, slightly noticeable at the third.
The highest setting is noisy, but we only need to run it on that one for a few hours after some allergen has accidentally been brought into the house, then we turn it down. It is a true HEPA, and does the work of a HEPA.
Filters do need to be changed every six months, but it is an easy job that literally takes less than 3 minutes.
The filters are not cheap-I shop around and get them for about $60 or so. But compared to other air purifiers we have tried and experienced in other homes, it's clear you get what you pay for.
If you don't have severe allergies, you don't need this level of solution. But if you do, this one can literally change your life.