First of all, a negative ion is an oxygen atom with an extra electron and completely odorless. Ozone is an oxygen molecule with three oxygen atoms. They are two totally separate entities.
A good ionizer actually will produce insignificant amounts of ozone at close range - way less than .05 parts per million. The best ionizers generate negative ions internally to electrostatically charge the filter. This allows more dust and bacteria to be trapped and eliminated from the air you breathe.
A great ionizer will work in tandem with a good HEPA filter to produce amazing results. Before getting an Aprilaire 5000 whole house electrostatic filter my ductwork still got dirty using a plain HEPA MERV 10 filter.
Whole house electrostatic precipitators clearly work better than plain HEPA filtration, according to Consumer Reports.
http://www.arb.ca.gov/research/indoor/cr-12-2007.pdf
Secondly, negative ions are extremely beneficial to have in/around your environment and counteract the affects of positive ions. Tons of studies exist on Pubmed demonstrating this fact. These filtration systems are used in major US hospitals for a reason.