If you have Apple mobile devices and are considering upgrading to the latest version of iOS (8.1.3), don’t. If you have upgraded, quickly downgrade back to 8.1.2 before Apple makes that impossible.
The reason is that this version closes one of the loopholes that was used to “jailbreak” these devices, and there is one very good reason to jailbreak at least your iPad: To install the f.lux app.
Research has shown that reading on backlit screens like an iPad in the evening is very bad for your sleep, because the light from the screen has the bluish-white color of daylight. Exposing your eyes to this kind of light in the evening confuses the body and lowers the quality of your sleep. The f.lux app compensates for that by changing the color temperature to a reddish evening light when the sun sets.
Unfortunately, Apple doesn’t allow apps to control the screen the way f.lux needs, so you will have to jailbreak your device to install f.lux. I strongly recommend you do so if you have an iPad.