O2 Sensor Extenders
 

Good or bad idea?

I currently get 12 mpg in my truck and was told they would increase that by about 13 mpg on highway (total 25 mpg highway) and about 8 mpg city (total 20 mpg city)

What exactly do these extenders do? Do they result in a loss of power or does it damage the engine in any way?

Anyone have these done on their vehicle?

i'm not sure about trucks, but for RSX's O2 extenders are just wires that extended your o2 sensor wires when your running a race header because the o2 hole is located further down the pipe

there are also Check engine light eliminators, witch is an add on pieces that goes on your secondary o2 sensor, tricking it to thinking that there is a Cat.
none of these do anything to MPG

It probably won't do what you want. Most likely is it leans your AF mixture, then since you know you have a gadget installed you will go easier on the pedal.

If you gain that much more MPG, chances are your engine components like O2 sensors etc need replacing in the first place.

Are you meaning a bolt in o2 spacer that extends your o2 sensor out of the direct exhaust flow?

yes

OK those are intended to reduce the exhaust flow over the o2 sensor for a couple of reasons but the side effect to this is reduced ability to monitor the afr.
Without an accurate afr reading the ecu will not be able to accurately calibrate your afr (air fuel ratio) the spacer most commonly contributes to a leaner reading so your ecu most likely will try to correct that by adding more fuel.
In other words there is no way an o2 sensor spacer will increase your mileage especially such grand promises of more then doubling it.

FYI we normally use them when deleting the cat and an o2 sensor is positioned after the cat.