The reason for why I should do it is because it is no longer as limited when its jailbroken, there is much more customization, much more awesome apps.
True, you can do much more with your phone when it is jailbroken.
However, I'd like to ask.. Should I really do it? Is there a risk of irreversably f**king up my iPhone?
Yes, that risk is always there. The risk increases by jailbreaking your phone since you will gain root access. If you do not know what you are doing, you can fuck up the device pretty good. If you stay with installing some well known stuff from Cydia, you should be fine.
I killed my Nokia x6 by installing corrupted Symbian firmware and nothing could fix it, even re-flashing the phone with correct firmware. Please, advice me?
Most of the time your jailbreak is deleted by a new firmware update. If you re-flash your iPhone, you will lose your jailbreak. If you fuck it up beyond recovery, you're in the blue.
If you're confident that you can stay focussed and don't do stupid stuff to your phone over ssh and delete some important folders, you should be fine.