This is my 2nd air purifier - the other was a comparable model by Holmes. I learned from my first air purifier that you really have to leave it on all the time or the revolving motor will collect dust, not work as well or as quietly. If I don't want it on constantly, I just put a clean towel over the top to prevent any dust/dirt from building up.
The noise level is acceptable to me, in an 8x10 room. I usually leave it on LOW, which sounds a bit like a fan on low speed, but MED is good too - it has a nice white noise effect. I only put it on HIGH if there's a bad smell or lots of dust floating around (for whatever reason). HIGH is a bit loud for my taste. This machine tends to rattle a bit as it sits on my bureau, but a small towel underneath it took care of that problem. Also, give it time to warm up when you put it on. It starts off slowly, but works at full speed within a few minutes.
I have allergies and noticed a difference immediately when I got this model. My symptoms have improved greatly, even with 3 cats in the house. I would recommend it to anyone who has a small space, doesn't want to buy new filters and is perhaps on a budget. Good machine at a decent price.