Is jailbreaking your iphone bad?

I'm thinking of jailbreaking my iphone 4, (cydia) and i was wondering if its bad? My sister did it, she gets unlimited music, cool themes, and her battery never wastes & few other things. What are the pros and cons?

No but while jailbreaking your phone might brick your phone. But jailbreaking your iphone you can get paid apps for free and you can cutomize your iphone

Jailbreaking is awesome, but it can brick your phone, make Apple employees hate you, and lose your warranty. I recommend doing it on a phone that has: a. No more firmware updates b. A broken/expired warranty. Free apps (vShare), free inapp purchases (iAP Cracker), and more await you in the free land of Jailbreaking!

Pros: you will be able to download and install pirated versions of games, cool themes and a bit more "freedom"
Cons: It won't be as simple to use all that, than it was before. It actually makes the device a little bit more laggy, which I don't like the most.