My girlfriend has three cats and an aversion to seasons, so I bought us a room-air purifier. It's a Whirlpool ap51030k, which I bought on Amazon for $300. I did a bunch of research and it got good reviews as a decent mid-level purifier. Being that I work in HVAC I have some knowledge of air filtration, so going into this I pretty much knew what these things do: They are just a box with a centrifugal fan that pulls air through a quality filter. I also know what they should cost (not much) and what it takes to design/make one work well (also not much). Now the first thing that you find when you start researching these things is that they cost several times more than you would expect. But, constrained by market realities, you look for a decent one at a price you are willing to pay. The next thing you learn is that 90% of air purifier review websites exist for the purpose of selling you their no-name brand air purifier and the reviews are fake. Because of this, I decided at the outset that I was going to buy a brand-name I knew I could trust. Names like Holmes, Honeywell, Whilrpool. Major manufacturers should produce respectable products and are less likely to fake their own reviews. So I found some review articles that looked trustworthy and also sampled the personal reviews on generic retailer sites (ie. on Amazon, not on airpurifiers.com), which were shockingly helpful (see the HDTV sound quality thread for the opposite). Here's what I found: 1. It is easy to recognize total scams if you aren't a complete idiot. For example: Since an air purifier is a filter with air flowing through it, good filtration requires a fan. Duh. My favorite, though: Plants are not air purifiers and even if they were, you don't have to buy a device to house a plant you already have. 2. Filtration quality varies widely, but since it isn't hard to make a quality air purifier, it isn't hard to find ones with good filtration. The basic principle is: bigger is better. Bigger fan and more filter surface area means more air gets filtered. 3. These things are loud. There's no getting around this and it is to be expected: High quality filters have high pressure drops and as a result, require high pressure centrifugal fans, spinning at high rpm to pull air through them. Now when I say "there is no getting around this," what I really mean is that you'll never get a centrifugal blower to sound like a ceiling fan on medium speed, but there are things you can do to make them sound less like a helicopter trying to land in your driveway. So you take sound into account when selecting. 4. Because it isn't enough to screw you over the purchase price, air purifier manufacturers screw you over filter replacement as well. My first choice purifier was a Honeywell 50250-N with a cleanable filter, but a review website (and many individual reviewers) warned me that not even Honeywell's website was clear about exactly which model they were selling. Seems one model has a cleanable filter and another one doesn't and they differ only by the letter at the end of the model #. So after an hour of looking for it, I gave up. But I'm not going to get arrested for cleaning a non-cleanable filter, so this doesn't bother me much. And that's pretty much it. Like I said, not rocket surgery. So I bought the Honeywell: It is "reasonably" priced and got good marks for sound and filtration. My first reaction when opening the box was: "What a piece of garbage. Did I really just pay $300 for this?" It is made entirely of plastic. There's more metal in the $15 space heater sitting next to it. And it's thin, flimsy plastic too. I could probably drop-kick my space heater down the stairs and it would survive, but I pray my girlfriend's cats don't knock this thing over. So I plug it in and turn it on. Yeah, it's loud. But it does have a neat little "sleep" button that puts it down to low speed for 8 hours whiile you sleep, then returns it to its previous speed. It also has filter life monitoring. Fairly basic controls overall, though. On closer examination, I look down into it and see that the fan appears to be wobbling. So I turn it off. Not only is it wobbling, but it is also out of balance: it slows to a stop, then rotates backwards and settles into a heavy-size down configuration. Unreal! (speaks well for the low resistance of the bearings, though I guess). After a few days of use, it's already making a ticking noise, indicating that perhaps a bearing is already failing. So I'm going to try to send it back and if I can't, balance/align it myself. I also see that it is equipped with sound attenuation. Nice. 'Course it is just a styrofoam baffle at the outlet and a styrofoam wrap around the back side of the square fan casing. With just a little bit of effort and money, they could have made a more aerodynamic case and lined it with some actual sound attenuating material and probably made a huge difference. So I don't want to oversell it, but again, this thing is crap. One would think that with a 97% profit margin, it would pay for a company with a reputation to put just a little bit of effort into quality, but nope: gotta squeeze that to 98%, particularly since I'm sure everyone else does it. If one were to consider a comparison between this and other room-level HVAC devices, it would compare pretty unfavorably to a room humidifier and be more expensive than a space heater if only because it's bigger. It really isn't worth more than $50 -- assuming that my out of balance/alignment fan isn't the norm. If I ever get bored and grumpy, I'll build my own and prove that it can be done cheaper and better. My girlfriend says it seems to be helping her alergies and is happy with it. So in conclusion, I would recommend purchasing the Whirlpool ap51030k to make your significant other happy and redirect your vomited frustrations over what a piece of crap it is onto random people on the internet.