I have two air filters that are on permanently: they're both Sharp FP R65-CX. The HEPA is a bit expensive, but we only have to change it once a year.
I'm a bit confused that y'all have to replace the carbon filters - mine are washable and they suggest rewashing and reusing rather than getting new ones, because after the wash they're more active than they are just out of the box. Every six weeks or so a little light will pop on saying 'wash carbon filter' - once you do, you press the reset button, and it won't bug you again until six weeks later.
The pre-filter is also washable - it's really just a mesh meant to keep hairs and other large particles out of the HEPA filter, as that would choke it pretty badly.
I used both machines on a single room a few years back when I was having a major, major, major allergy flareup - anytime I was outside I had to walk around with a N-95 mask on (it was fall, there were airborne molds because of the leaves, plus my adrenal system was almost nonfunctional), but the air filters let me not have to wear a mask when I was in my residence room, which was far more comfortable and convenient than having to wear a mask to bed!
Mine have several settings - I usually leave them on Max, but there's pollen filtration, silent, medium, max, and there are timer programs (or energy saver, I guess they're called). They also have built in humidifiers if you need to humidify the room (but they humidify it without heating the water, which is very nice). I don't mind the noise at all - it's really just a soft shushing white noise. If you're listening to your laptop speakers without having headphones plugged in, the white noise can make softer words tricky to make out, but when that happens my boyfriend (I always use headphones) just turns it on silent running, and goes back to listening. Honestly, our air conditioner makes more noise than both air filters combined and then tripled. It's great in an apartment too - that way you don't have to hear every word the neighbours say!
