If you listen to the sales people, this is a hot debate. If you talk to Health Canada, this is a settled issue. Isabelle Sauve from Health Canada was as clear as the warnings issued by her department (January 1999) -- devices designed specifically to produce ozone may be dangerous to your health and should be disconnected! Health Canada, along with the Canadian Standards Association (CSA), has even taken the necessary measures to eliminate from the market all such devices that plug into the wall. The only legal ones left in Canada are battery operated, and are considered just as dangerous.
However there are commercial ozone generators used across the North America to clean up air quality problems. What is the issue? Essentially it is quite true that ozone in high enough doses can kill bacteria and generally clean out air quality problems. The problem is that when it is generated in high enough doses to accomplish that, it will also cause irritation or damage to human nose, throat, and lung tissue -- in fact to any living thing. Ozone is very corrosive. So in commercial applications it is a good cleaning tool after a fire, or after some kind of contamination -- but people are taken out of the building during the treatment and the building is aired out before they come back. These devices should never be operated with living things around. (Actually any motor generates a little ozone all the time because of the electrical sparks from the brushes, but these concentrations are far too low to cause harm in a house, or to clean the air.)
To read Health Canada's warnings, go to www.hc-sc.gc.ca/psb or www.hc-sc.gc.ca/english/protection/warnings/1999/99_62e.htm.
Ozone generators are not to be confused with Negative Ion Generators, which work very hard to minimize or eliminate how much ozone they generate. The Negative Ion Generators do help to clean the air with no bad side effects and I support them strongly.
Wondering when or even why you might want to clean up your air? Check out "Air Quality" in the Nuts&Bolts section.