Do programmable thermostats save money or do they ruin your unit?
They won't harm your unit. In fact, they may protect your A/C compressor if you don't already have a time delay to prevent starting it again too quickly after it's turned off. Almost all A/Cs today have this delay.
If you use it you will save some energy/money. How much you'll save is a difficult question. You'll probably save more with heating than air-conditioning. Whether you'll save enough to pay for the unit while you're in your current house becomes the issue. Programmable thermostat manufacturers tend to exaggerate the saving your will get (Bet you're surprised :).
Apparently studies show most people either don't program the setbacks, or override/defeat them with "Hold" settings. Based on this you will find articles saying they don't save money. Sort of like studies showing smoke detectors don't save many lives because most people don't replace the batteries, so they don't alarm. It has to be true -- if you don't use something it won't help you. So, if you don't intend to use it, don't buy one. I'm guessing people stop using them because they can be annoying/uncomfortable. If you program it to start the setback too early in the evening you will have to get ready for bed in the cold (or hot in the summer). If you program it to set back after your sure you'll be in bed, you must either get under too much cover and be too hot initially, or wake up freezing and cover up. If you have to get up in the night it will be cooooold.
It's something of an over simplification, but how much you save depends mostly on how much setback you use and how long your house stays at the target setback temperature. Thus, one long setback period will save more than two short ones totaling the same amount of setback time. The worse your insulation is the more you will save, and vice versa. The poorer your insulation the faster you lose heat, and the quicker the temperature will reach the setback temperature. Thus, the more of the setback period it will be at the setback temperature. If your insulation is very poor, investing in better insulation will save more than an automatic thermostat.
Two brands that have gotten good ratings in the past are:
I've been told Lowe's has a good selection, and here are a couple of places you can find programmable thermostats places on the net. I'm sure there are many more.