Some questions from the newb

I'm thinking about jailbreaki my iPhone 4 but I'm kind of sketched out, not sure of all the risk (I've hacked my itouch but never my iPhone)
If I've done my research right there is a way to basically have truly unlimited data through cydia, but won't at@t notice all the data I'm using? What causes your iPhone to "brick" and is there really anyway to fix that? Are there any other risk or anything I should know about? Nwas the jailbreak worth it to you!? I love my iPhone 4 and want to make sure I don't do anything to ruin that.