Just changed the AC filters in the house today. I use the 3M Filtrete 1700 series as I tend to have allergies and these are rated best. Only thing is that they are $20 each and I have to replace three: two 20x20x1 and one 16x16x1. 3M recommends changing these at three month intervals and the ones I changed today had four months of use on them and they were very dark brown and smelled. Glad to know that these filters are doing their job! Just wondering what others are using or any suggestions.