I just bought this unit for the third time in 20 years. In the mid-90s in Brooklyn, I bought it to help with cat allergies and kept it for 6 years. In the mid-2000s in Chicago, I bought it to help with dirty downtown-apartment air and kept it for 5 years. Now, I have it once again in a Chicago apartment to help with a "sick air" residential building (dust/mold/dirt from hallway and facade renovations.) It has never failed me. Like the other 80 percent of reviewers giving four or five stars all say, it just works. Like, works in the way that products used to work 20 years ago.The people complaining that it's an old, bulky design that's a little louder than more modern units? Good! The last "more modern" unit I had--a Holmes mini-tower HEPA purifier--allowed so much air to bypass the HEPA filter, it spit out black soot on my walls and floors. It ended up on a one-way trip down the garbage chute after two years of my partner and me wondering why we always felt sick and congested even though we ran the unit 24/7. I opened up the unit and found everything inside caked with thick black gunk from all the bypassed air.Yes, on Turbo the Honeywell 50250 sounds like a 737 ready for takeoff. But you only run it that high when you really need to, which is rare. On low it still sounds like box fan on high. But just like running a box fan, you tune it out pretty quickly. Eventually, your house comes to feel weird without the noise of the unit in the background. Most of all, you feel an immediate change in the air. This unit, even on low, puts out enough clean air to create a breeze that will flutter your clothing as you walk by. If you live in a "sick" building like we do, you will notice the difference every time you enter and leave your front door, and it's a big difference.You do have to replace the pre-filter (which is relatively inexpensive) every three months. You can cheat and wash it like I used to, but the charcoal just washes away eventually. You really do want to replace it. You can carefully vacuum the outside of your HEPA filter but you won't need to replace it--which is good since it's more expensive--more than every three years or so. Most important is to keep the outside of the unit clean/vacuumed and replace the prefilter. A dirty prefilter you refuse to replace will make the machine become very loud and very inefficient. TRUST ME on this--I've been there too many times in the many years I've owned this machine. Do yourself a favor and just buy a few pre-filters in advance.The machine feels all 20 of its pounds heavy, but the handle on top makes it manageable to move, or to carry from room to room. We will eventually get Honeywell's small (sub-100 sq. ft.) True HEPA machine for our bedroom, but for now we split the machine's time between living/dining room and bedroom. A couple of hours in a new room is enough to make it feel like brand-new room. (I can't stress enough what I wrote at the top of my review--this machine really, really just works, and works alarmingly well.)I've added photos of the unit and its box so you can get an idea of its size and bulk relative to normal furniture (and normal-size adult cats!) In the photos, you can also see what the inside of the unit looks like when it's open, top and bottom, as well as see how the pre-filter wraps around the HEPA filter and is secured with two Velcro strips.I can't speak to power use, but even on low the machine is noticeably powerful. Over the years, we replaced all our incandescent bulbs with compact fluorescents and LEDs, so it will be apparent pretty quickly if our electric bill goes up significantly. I may come back to this review in a couple of months to revise it with the change we notice in our bill from running the machine on low 24/7.And, please, don't be silly. The machine was designed to run 24/7. The instruction booklet suggests you run it 24/7. If you want all the wide and nifty health benefits of clean indoor air (cardio-pulmonary, cognitive, attention-related, stress-related), run it on low 24/7. I only ever turned my previous two machines off to change the filters. Then again, you'll probably come to love the device so much that not running it will feel like leaving your house without your cell phone.Having this machine again is like seeing a beloved old friend again. I can't believe how much I missed it, or how much better the air is in my apartment already. HIGHLY recommended!!!