For the most part, softeners are pretty simple. Softening water works by exchanging positively charged ions with sodium ions.
Because softeners backwash periodically to recharge, they have a timer (and sometimes a meter) to control when this happens. One of the problems with a mechanical unit is when the power goes out, the time freezes, and just picks up where it left off when the power comes back, so it will start backwashing at strange times - most (if not all, now) electronic timers have battery backup for the time. Electronic ones have been in use for 10+ years now, and are quite reliable now. If you use a metered timer, it will regenerate during the night only after using a certain amount of water, so it wastes less brine (salt) water.
Get one that is easy to service. Although softeners only have to be rebedded about once every 10 years, it should be possible to turn a bypass valve and disconnect the softener. I've seen some old units where you actually have to cut the plumbing to disconnect them, although I'm not sure if those are still on the market anymore. You don't want one of those.
Pay attention to how it is connected: All of your hot water should be soft. Outside taps should be hard (you don't want to water your garden/grass with soft water), with perhaps the exception of a hose in your garage for washing cars. Some people leave their kitchen cold water tap hard, so as to reduce the amount of sodium they are drinking and cooking with, but this partially depends on preference and partially on the amount of hardness (and thus amount of sodium being added). Obviously this is a bigger concern if there are people on low-sodium diets in your house.
Personally, I would avoid electronic/magnetic softeners. There has been some evidence that this can help with very low levels of hardness, there is a LOT of junk out there.
I used to work summers for my dad who ran a water treatment business. We replaced one of these electronic softeners (it had wires going from each end that wrapped around the pipes) with a real softener one time, and took the electronic one back to play with. We tore it apart, and could not find any active electronics in it except for the ones that drove 5 LEDs on the front that flashed back and forth. We hooked the wires up to an oscilloscope and all it was doing was alternating between about +10 mV and -10 mV, with almost no current. I can't see how this possibly did anything -- and the home owner was replacing it with a real softener, so what does that tell you..
Best Answer
The setting is the "input hardness". The zeolite matrix resin (the part that does the water softening) will nearly completely soften the water until it runs out of sodium. The resin in the tank is good for a certain amount of "hardness" (measured in grains), probably in the range 10,000-50,000 grains. The setting is used to calculate when the resin will need to be refreshed (by measuring the quantity of water delivered, or using your estimated water usage).
If you input too low of a hardness, you'll have softened water for a while, then you will start to transition to unsoftened water. At some point, the unit will refresh itself, and you'll have soft water again.
If you input too high of a hardness, it'll refresh too often, wasting water and salt.
There's no "standard" hardness, it really depends on your water source. As an example, where I am in Indiana, it is about 18 grains/gallon of Calcium. If you have iron in the water, that must be factored into the hardness, usually at a rate of 4 grains per ppm of iron.
If you use municipal water, they probably publish the specifications of the water you receive. If you have a well, then you should have your water tested, preferably using a titration method (the paper strips work very poorly for measuring hardness).