We bought a FLEX for our relatively small bedroom (about 200 square feet) and it works extremely well. We run it at high speed when we're not in the room and at a low speed when sleeping. The unit is rather loud at high speed, but the sound is diffuse and not at all unpleasant. We vacuum the pre-filter every week or two. We do not like the immediate appearance of Alen ads on every website we visit, but this seems to be part of the price for anything you buy these days.