Ionizers simply negatively charge the air's so that floating dust particles will cling to any nearby surface in the room, much in the way static electricity does. In fact,

Some ionizers do have collector plates to collect the dust, but these collector plates typically collect less than 5% of the total particulate matter. 

Most of the dust just adheres to the nearest possible positively charged surface in the room, that being your walls, floors, and ceiling.

The problem with this is that the same dust can be kicked right back up into the air. 
The dirt that does not begin to float again will stick to whatever surface is close. 
After a few months walls, floors, appliances, and furniture begin to build up a layer of grimy dirt on them. 

If you put an ionizer on near a wall, the wall will develop a black halo of dirt in just a couple of weeks.

Six months after you start using an ionizer you will have MORE dust and particulate matter collected in your home than before you started! 

 With a high quality HEPA filter you can collect the dust and throw it away, rather than saving it on every surface of your home.

For removing airborne particles of dust and dander a HEPA filter is much more effective than any ionizer can ever be. HEPA filters can trap 95% more dust than any ionizer collector plates. HEPA filters collect the dust and dander rather than just push it around the room as an ionizer does.

With an ionizer the dirt that was just floating around over "here" a minute ago is now floating around over "there". That is no solution.

HEPA filters requires no cleaning or maintenance to maintain this efficiency.  In fact HEPA filters actually become more efficient over time filtering out smaller and smaller particles since as the filter gets filled, less large particulate matter can pass through them.

To clean the HEPA filter all you have to do is vacuum the dust off of the prefilter on the outside of the unit.

Because all respirable particulates including bacteria, fungi and other minute airborne contaminant's are trapped in the HEPA filter, the use of HEPA filtration, will significantly eliminate or reduce the health effects of airborne contaminates on the respiratory system. 

It is for this reason, that HEPA filters are recommended by more allergist's and medical professionals than any other filter system available. 

Ionizers also produce a very small amount of ozone which can contribute to a fresh spring like smell.  The electrically charged air produced from an ionizer can smell very fresh.  But that does not mean that the air is clean. 

Ionization's fresh smell is a cover up, not a cure.
If there are odors in the home and that fresh smell is desired it is recommended that rooms be first blasted with a high powered ozone generator. This will kill the bacteria and the odors. 

It is very simple, ionizers are VERY cheap to make.  Most ionizers, including the $350.00 one you see on TV, are built with no more than $30.00 in parts! 
The company that is selling them on infomercials is having the greatest success with their ionizer than with any other product they have ever offered. 

They are also making more money with this product than with any other item they have ever sold.  It only makes sense for them to spend lots of money to promote this product on TV and the internet.  Unfortunately they are not very good air purifiers.

Ionizers don't kill any organisms, therefore they will have no effect on airborne mold spores. 

ONLY a true HEPA filter can collect mold spores. Airborne mold spores, even dried ones are very dangerous to breathe.  Collecting these spores is essential to maintain a healthy environment in your home. AllerAir Ultraviolet HEPA units can even kill mold spores floating in the air.

Please note that if you have mold in your home that air cleaning is NOT enough to solve the problem.  It is recommended that as much of the mold be removed as possible, and that unoccupied areas be treated with high doses of OZONE in order to kill the mold.

The Permafilt™pre-filter traps larger particles such as dirt, lint & hair. 
This filter can easily be vacuumed from the outside.
15 lbs. of carbon & zeolite mix absorbs odors & gases -with a 'Miracle Inch' of complete protection.
True medical HEPA filter paper removes harmful airborne particles. 
The HEALTHMATE ™ filter removes over 3000 odors and gasses along with 99.97% of all airborne particulate matter measuring 0.3 microns or larger.