Salt-free water conditioners can be a great alternative to water softeners. Though they don't physically remove hardness from your water, they do an excellent job at preventing hardness minerals from forming scale deposits, and they do it without the use of sodium.
This means you'll need no post-filtration and will experience no salty tastes nor the “slipperiness” commonly associated with soft water. One customer recently asked whether a salt-free conditioner could also be used to prevent scale build-up and preserve the membrane in his RO system.
The answer, unfortunately, is no. A more effective method of preserving your RO membrane is to inject anti-scalant into the water before the RO system. This simply requires a metering pump and solution tank, and an anti-scalant such as PreTreat+ NSF. Just add the anti-scalant to the solution tank, and it will be injected into the water by the metering pump.
While PreTreat+ can be a bit pricey, one 5-gallon pail should last 1-2 years in most residential applications, and will dramatically increase the life of your RO membrane, far longer than a salt-less water conditioner would.