Is Jailbreaking bad for your iOS device?

I'm never going jailbreak my 4S but I've heard rumours that Jailbreaking is bad for the iPhone and iPad and iTouch?
What's the point of it and what does it do?
How do you jailbreak?
Is it illegal?

Pros: You can access tweaks like change your theme or download free apps through Installous
Cons: Void warranty and may lag >.<

Personal Experience:
Well i mainly jailbreak my iPhone because alot of Apple apps are mostly paid, while since Apple has LOADS of restriction, i can download tweaks like Color Keyboard to change my keyboard theme, or maybe add more security by using ICaughtU. If you just got your iPhone don't jailbreak it unless your warranty is over.

Its not bad for anything. It works great. And its not illegal, apple just doesn't like it. Why wouldnt you jailbreak it?

oh and it voids your warranty only if they see it's jailbroken.restore your phone to normal if you go into the apple store.