What sort of things should one look for in a water-softener?
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..