A more recent list of problems:
1) Overpriced replacement filters
2) "Washable" filters that do not deliver results
3) Loud Air Purifier Fan Motors
4) Does not remove dust or pollen as promised
5) Air Purifier Limited Warranty Scams
6) Poor Customer Service
More problems listed below...
CADR Does Not Factor Germs, Bacteria, Mold, Mildew, Fumes, Odors, etc.
The CADR is basically only a measure of how rapidly air is circulated through a given air filtering device. While this figure is perhaps relevant for the vast majority of filtration-only devices that attempt to clean air using only one or two types of filters, it's meaningless because filtration is simply one single aspect. The fact is that the CADR number tells you nothing about how well, or even "IF," an air purifier filters germs, bacteria, viruses and other harmful biological agents -- nor does it assess how well the unit clears chemical fumes and odors. The easiest way to think of it is that, even though everyone in the industry refers to their products as an air "purifier," the fact is that the overwhelming majority of competing products out there are simply air "filters" that don't use anywhere near the number of additional "purification" technologies available, e.g., UV light, TiO2, activated charcoal, negative ionization, etc.
HOW TO EASILY MANIPULATE CADR NUMBERS
The fact is, if a manufacturer wanted to market their purifier just according to the CADR numbers, one could simply 'beef up' the fan speed, eliminate certain technologies and just manufacture the same limited filtration systems most companies use. For example, if you pull air too quickly through the unit in order to artificially inflate your CADR numbers, any UV technology that protects you and your family from harmful germs and viruses that traditional air purifiers miss, might negate any effects of killing germs and viruses.
THE POLITICS of CADR
I was doing some consulting for a company who asked me about CADR and how to use these readings. It turns out that the AHAM organization (American Home Appliance Manufacturers), a political lobby group, is the organization that does the testing and certification behind CADR. It costs an air purifier company $10,000 per year to be able to use CADR in their manual or on their website. The AHAM organization represent big companies like Honeywell and Whirlpool to 'seeemingly' make sure politicians in Washington support their agenda. (I'm sure there are some good lobby groups, but I get a little leery of lobby groups and what they have done to world views of America, to the environment, let alone to consumers. I've heard a few stories about AHAM which I don't like).
CADR will give you some idea of how the air flows across your HEPA filter, but that's about it. It won't give you any idea of the purifier's effectiveness in eliminating odor, gas, viruses, bacteria, etc.