Smoke/dust and other small particles are attracted to the high voltage probes (Dont poke a multi-meter or finger on them) so they do improve air quality to some degree. As for beneficial effect of "negative ions", reams have been written by people both for and against.
I have a buddy who recently spent about $400 to put ionizers in his whole house, one for each room. I took the liberty of taking one apart today to check it out. The ones he got put out a pleasent bleachy kind of smell which I suspect is partially the ozone. As far as their effectiveness I'd say they work quite well at making his house smell like it just got rained on. I'm very tempted to get a few myself now.
Regarding the ozone output, that will depend on the unit you buy. SOME ozone is good because it will kill stuff and oxidize contaminents in the air (oders, hydrocarbons, etc). Most ionizers work by creating a static discharge in the tens of kilovolts range. The higher you go, the more ozone it makes. If the unit was designed properly (read: correct voltage output) then the ozone levels should be low or at least within human tolerance limits. If you are especially sensitive to ozone then you might not be happy with any common ionizer.
I intend to grab a couple of the bare kits from Electronic Goldmine to experiment with.
Which I think is very humorous, because the air ionizers you see on TV do this too, so they always have a warning saying something like "Use in a well ventelated area"
Well if it was well ventelated... there wouldn't be dust in the air, same with mildew remover, if there was a well ventelated area to use it in, there wouldn't be mildew.