Its permanent (but can be changed anytime). And yes, you can screw your box by installing this. But you can backup your eeprom data, HDD Key and HDD content first. With this information/files you can always "repair" your box.
on hdd and doesn't get loose if you shut off or change games.
But right, hardware (beside hdd) isn't changed in any way (if you shut down the box) and everything can be restored to original state (if backup'ed before).
Start with the pinned tutorials in this forum and also look at Krayzies signature, there are some more essentials.