have been running my air purifier 24/7 for almost a month now. Got this right before Christmas. I haven't had any problems with this unit.

PROS:
1) Really cleans the air- after just a WEEK of use, the outer (carbon) filter was covered in dust! Disgusting, but I'm satisfied to know that wasn't in my lungs. I vacuum the filter once a week.

2) HEPA filter. Although I can't actually "see" what it is catching, I know it is helping my air quality! I live in Utah where the inversion pollutes the air on a regular basis during the winter. This air purifier gives me peace of mind, knowing that when I come home I am in a clean air environment. Not only that, but it catches dust mite allergens, pollen, plant spores, bacteria, fungi, mold, smoke, dust.. We are definitely sleeping better at night, not waking up with the usual "stuff" that you have to clear out of your throat in the morning catch my drift?

3) UV light- peace of mind that the germs that are caught by this thing are killed.

4) Catches SMELL (carbon filter)- I am used to smelling leftovers when I let my pots/pans soak in the sink overnight. Not anymore! I know this works because one time we went out of town overnight and turned the unit off. When we came back, our apartment smelled like soaking dishes/leftovers. A smell I'm glad to be rid of using this unit!

CONS:

1) When I first got this unit, the lowest setting was pretty quiet. It is noticeably louder now. However, at this setting me and my husband are still able to sleep with the unit in our bedroom at night. AKA not too loud for sleeping (and I used to sleep strictly with earplugs for 2 years straight, so that should tell you something!). Because the fan got louder, I rate this product 4/5 stars. I hope it doesn't continue to get louder. I will update if it does.

2) When we first used it, it did have a "new" or plastic/carbon smell to it. I imagine this was the carbon filter. The smell was gone by the second or third day.

I recommend this product. It is the best Air purifier/HEPA filter you can get for this price range. Obviously if you spend more money, you can get a much better product that will cover more square footage.

**IMPORTANT- the 22" model can be used up to 155 sq. ft. The 28" model can be used up to 193 sq. ft.
Buy according to your needs. AKA, measure the living space you want to use this unit in. In bigger rooms you will need MORE THAN ONE UNIT if you want this product to work effectively. Just an FYI.

******UPDATE 1/17/14 *******

I contacted customer service about this air purifier becoming louder, and this is what I was told:

"We are sorry to hear of the issue with your AC4825. If you have checked inside of the unit to make sure there is no buildup of dust/debris, and if the filter is still good(the white part is not brown or black), then you can send the unit in for warranty service. Since your product is in need of repair please open the attachment named, "In-Warranty Repair Form". After printing the form, follow the instructions on the form in order to have your product repaired.

4 Simple Steps To Returning Your Product For Repair:

1. Print and fill out the applicable Repair Form attached (In-Warranty Repair Form)

2. Pack your complete product (including all parts) in a 16x16x16 box or a smaller sized box for lower cost shipping rates.

3. Send us the applicable Repair Form inside your product package along with the following:

a. Provide a copy of your sales receipt or rewards history (REQUIRED)

4. Mail your package to us via UPS, FedEx, or USPS. We recommend you have your package insured.

Mailing Address:
Guardian Technologies
Repair Department
7700 St. Clair Ave.
Mentor, OH 44060

Once received, your product unit will be inspected and repaired and our technician will contact you with information on your repair. Normal turnaround time is 10-14 business days."

So basically if I wanted it to be fixed, I would have to mail it in myself (and pay for shipping). Not the best customer service in the world. I've dealt with several companies who would just send a new unit out (so long as I provided proof my unit would be disposed of, AKA cutting the power cord and sending a picture of that + unit/serial number).

I'm NOT going to send it in because I know shipping will cost 1/3 what I paid for this product. No thanks- I'd rather buy a replacement filter for that cost.

******UPDATE 2/22/14 *******

Replaced the filter and the sound is back to normal- quiet on the lowest setting! I guess our place is pretty dusty so it got dirty faster than usual. I recommend anyone with noise problems to replace the filters more frequently! This will make a noticable hole in my wallet seeing as I have to replace the filter every 2 months when I was expecting 6 months at least.

I still recommend this product highly! Thanks GermGuardian!