...and learn how they workBefore we begin, I just want to point out that I'm not an expert on this (Lloyd is). This is just my story and a few of my thoughts. I know there's a lot of controversy about programmable electronic thermostats; like the VCR of old, few people know how they work and are able to get the most out of them, so a lot of their theoretical benefits go unused (as Lloyd pointed out). That's why the NEST was created: It might be expensive, but at least it's easy to use and intuitive enough that you're very likely to end up saving energy with it.
Hopefully NEST will have a similar impact on the thermostat industry as Apple had on the phone industry, and soon all programmable thermostats will be easy to use and effective at saving energy...
Back to my storyBut that's a different discussion. The reason I'm writing is because I've decided to replace our old antiquated thermostats even thought I rent an apartment (with the landlord's permission and help, of course). Where I live, 100% of the electricity comes from hydro and most people have electric heating (and it gets quite cold in the winter). This means that having precise and programmable thermostats can save many time their cost in electricity in a relatively short amount of time (we pay the heating bill), with the added benefit of being much more comfortable than with the old ones that make you guess what temperature they are really set at, and that constantly overshoot and undershoot while cycling on and off.
Because I work from home, I did not get programmable models. I just manually reduce temps before going to bed and bring them back up in the morning. But most people who work outside of their home and follow a regular schedule could see even bigger benefits by setting things up so that less energy is used when they are out of the house.
The difficulties with programmable thermostats might be widespread, but I hope that if you're reading this, you'll be different. If you buy or already own some programmable thermostats, take the time to figure out how they work and use them to their fullest. Maybe you could even offer family and friends some help to figure out how their thermostats work, or to program them for them. That's a nice green gift to give in this holiday season (the coldest time of the year in the Northern hemisphere, so a perfect time to save on heat).
Some thermostat resourcesThe U.S. Department of Energy has a primer on thermostats, a short guide on how to program a thermostat, including an interactive tool and some more rules of thumb and guidelines.
Consumer Reports has useful a thermostat buying here.