Here Are SOME Advantages & Disadvantages...
Advantages:-
-Access more stuff on your device like themes etc.
-Download paid apps for free easily
Disadvantages:-
-MAY slow device
-Your device warranty goes
"Jailbreaking" is another word hacking. Therefore you void your warranty but you get cydia, a jailbreaking software. Jailbreaking does have it advantages and Disadvantages. But if you want to send it to apple to repair it you have to restore it to factory settings to get back your warranty.
By jailbreaking it.
Jailbreaking will void your iPhone's warranty.
You cant without jailbreaking it.
Jailbreaking is free.
Jailbreaking a phone is partially legal and partially illegal. I have a post regarding this http://www.techlucid.com/2013/11/is-jailbreaking-illegal-or-not/
Yes and it works pretty well for me
yes it is absolutely legal
It is okay to jailbreak your iPhone 3g. The only downside to jailbreaking your iPhone is that you will loose your Apple warranty.
Yes by jailbreaking your apple device
No. Sorry.
It is not illegal but will invalidate any warranty.