I have dust mite allergies and when I lived in NYC, we had 2 expensive units. But they apparently were the wrong kind. They released Ozone? I forget. I ended up getting rid of them.
They did help though.
I bought a new one a few years ago (HEPA, no ozone release) because sometimes there are fires in SoCAL so I need to clean the house air. It totally helps.
I haven't used it years though because I just don't need it anymore.
The best thing I did was invest in expensive dust protection mattress covers and pillow covers. The priciest ones last longer and hold up the best.
I have a weekly cleaning woman and that helps a ton also.