Good Reverse Osmosis (RO) and Distillation units can produce almost distilled-water quality. Unfortunately, for "bad" water with high bacteria count, high minerla cointent or iron or sulfur problems, they are also the units that clog off the fastest, so become an operational nuisance and tend to have short lives.
If you are looking to remove specific chemicals from the water one of these units may fill the bill, for a general hard water or iron buildup or sulfurous bacteria removal, generally you would need a high-end multi-stage unit to equal the performance of a normal sand filter and resin exchange tank unit (the typical "Culligan Man" unit).
Click on the Home > Water Treatment subcategory in the Browse Project area at lower elft for a lot more discussion in response to questions about salt-free units and water treatment in general.
Also, google this search term and you will find a couple of Angie's List articles on the subject, and at leadt one video - water treatment angie's
