Water filters, reverse osmosis machines, and water distillers are effective at getting most of the stuff that isn't water out of your water. The problem is that these methods don't even address the issue of the pH of the water. Consequently, we often are innocently consuming large quantities of acid water thinking that we are finally doing something right.
Water ionizers are useful for changing the alkalinity of the water to a high alkaline value like 7.5 to 8.5 thereby helping your body to wash out acidic crystals, fat (acidic lipids), and help neutralize free radicals. Also, alkaline water tastes "thicker" because of a stronger surface tension. The higher pH water also is more able to dissolve oxygen, so you can opt to bubble O2 or O3 into it as well.
The best cheapest model that I could find is called the Jupiter water ionizer. It is made in Korea, but it is sold all over the place.