For hardware (something such as a Tamagotchi, or a DVD Player), usually I flip through the instructions, just glancing at the pictures. If the device looks intuative, I give it a go until I get stuck, then actually read the instructions.
For software (such as a computer game, or video game), I ignore the instructions until I get stuck... usually this is fairly on in the game - since I need to know which keys I need to press to run, duck, jump and so on. On the other hand, if the game starts off with a tutorial I might not need the instruction manual at all.