Recently we purchased the 75i and what a difference it has made in our home. My first purchase was the 45i and I realized I needed the large unit. The Alen customer service representative was the most customer oriented person with whom I have ever dealt. He made the exchange so easy. The unit is quiet and takes up very little space. Having allergies and asthma, the Alen BreatheSmart keeps our air cleaner and I really am breathing better. I give this unit and the Alen Company an A+.