Not sure what your main goal is. General health and eliminating colds, getting rid of dirt and dust, or pulling allergens out of the air? It's a huge market and everyone's getting into the act, from purifying air to keeping allergens at bay through mattress covers and pillowcase covers.
Like most machines, they can get you on the initial price, or they give you a low price and then really get you on the replacement filters (sort of like free or cheap printers - then they get you on the high-priced ink cartridges!).
If you don't change the filters, nothing is going to work very well. I have mixed feelings about all of them, although we have always used one for "white noise". I think dust is a part of life. I also know that so many things give off toxins, from our carpet pads to the foam in our furniture, so a purifier isn't going to do a thing about those.
We had terrible allergies, and we thought a purifier would work - we spent a lot of money on having one in each room, and a whole lot of hours cleaning out the insides, rinsing what could be rinsed and paying to replace filters that couldn't be cleaned. I'm pretty sure that the process of cleaning/replacing released as much stuff into the air as we would've had if we had no filter to begin with. Truthfully, the only way we got results was by strengthening our immune systems and just not reacting anymore to those substances that had triggered allergies in the first place. Most things in nature are really harmless, but abnormal changes have occurred in our bodies that cause us to react, or overreact, with oversized and miserable symptoms. A filter doesn't address the underlying cause, just the symptom. We had much greater success when we addressed the cause.
So it depends on what you're trying to accomplish. Is it a general feeling of "I want to breathe healthy air"? I'm not sure filters help much - the air does exchange with outside air, and it should. Breathing the same air over and over is not good (look what happens to people on airplanes breathing recirculated air). And it's impossible to achieve purified air in a home.
None of these machines will take out harmful chemicals that you breathe in your day as families go to work, school, activities, stores, etc. - that's what we need the EPA for! As for germs, certain viruses flourish in warm, dry environments which is what they get in the winter in a heated home in periods of low humidity. Nothing you can do about that except change your body's response to them.
