I bought the same Honeywell air purifier as pictured from craigslist for about $40 from someone who was moving. I kept it in my home office (when I had one) because that's also where the cat tree and litterboxes were at. It helped keep the litter dust and airborne fur to a minimum...but of course it needed cleaning about once a week. I felt it was worth it because I didn't pay retail for it :)

I have one of the heavy duty Blueair filters in my bedroom. It was expensive, but it sure does work! We all have allergies to dust to and pet dander, and it keeps it all at bay. A friend with terrible dog allergies came over once, and so I put the unit right next to her. Her coughing and wheezing were gone in a matter of minutes.

My wife suffers from allergies and got the Honeywell air purifier (looks just like the above image) for our bedroom. She says it's made a huge difference for her. I personally don't like how loud it is and how bright the light on it is, but I'm a light sleeper. A little tape over the light and I can bear it.

It depends on what the issue is for you. Smoking, allergies? I lived temporarily with a smoker and it made a massive difference. Without it I was wheezing and snoring but with it I was OK. I live now with animals, to whom I'm allergic, and it makes a massive difference. I use standalone air cleaners but as I'm renovating the house one of my wishlist items is a whole house air cleaning system (about $750 installed) for which I'd have to rinse or replace the filter monthly. For me, at any price, it makes a big enough difference that it's worth the expense.

I got a Blueair while temporarily living in LA and gagging on all the air pollution. It was expensive, but worth it - the smog bothered me when I left the apartment, but inside, I could breathe. I slept like a baby after getting it, too.

I use white masking tape to cover the power light and set it to low while sleeping. Look for HEPA filters, which trap pretty much everything you shouldn't be breathing.

I bought a winix hepa air filter after lots of online research...it costs about ~$150, the air filter is changed once a year and costs about $50. Before the filter I was spending $40-100 a month on medications just to keep my allergies at bay...now I don't even bother. To me it sounds like as if the AC is on, I have gotten use to it and now sleep with it on. It has been a lifesaver and I am thinking about buying another one for the other room!

I am saving up for a Honeywell for the bed room but I have some cheap ones in the living room, basement, and bedroom currently and it has made a difference. I still have allergies - because I insist on keeping the windows open. But my biggest allergy problem in the house is dust and dog dander, the cheap ones have helped with that enough to make a difference - but the trade off is the expensive filters that need to be replaced every 3 months.

after two years with the cheap ones we realize we should have got a good one with a washable filter and it would have paid for itself by now.

if you have allergies, then YES! if not, i'm not sure how much you'll notice what it does. they're mainly to filter out detritus to minimize allergies.

also, say if you have pets [that you're not allergic to] and you have a friend coming who is allergic, you can run it in addition to any normal prep you might do. my friends have always said they don't feel allergic to my cats, and i think the purifier helps in that regard.