If you use a fair amount of water and need it often, and have reasonably hard water, then the larger systems are what you want. Those other posters have mentioned.
If you occasionally need a water softener, or not real hard water or not very often, then look at the "waterstick" It's a smaller very easy to recharge system. That's what we use. But it has about 1/3 the capacity of the larger systems.
watersticks.com.