Honestly, the mileage you get out of air purifiers depends heavily on the region in which you live, the quality of your HVAC system, whether you live in a house or apartment (and its age), and whether you have pets. I live in one of the worst places in the US for allergies -- the Ohio river valley -- and depend on mine for about six months out of the year.
The marketing is in pushing consumers to buy bigger purifiers than they actually need, and/or more complex ones that do the job at higher price points. The rub is if you look at the small print on most commercial air purifiers, their room size recommendations are based on the cubic feet of air the purifier can circulate five times per hour. That's massively overkill, unless an inhabitant has COPD, asthma, or some form of autoimmune disorder that makes clean air an absolute necessity. Most air purifiers will easily keep spaces up to 2-4 times their advertised size comfortably clean.
You really don't need a multiple-hundred-dollar app-driven purifier with internal air quality sensors, with electrostatic and HEPA filters, that circulate air in a given room 3-5 times per hour. Your home isn't a clean room, and it certainly doesn't need to meet or exceed FDA standards for medical and food facilities.
The bottom line is if you live in a new-ish domicile with an up-to-date HVAC, as long as you're not in an area prone to air quality issues, you probably won't get much mileage out of an air purifier. Just keep up on home sanitation.
