It can be bad for your phone. It does slow it down a bit, but nothing you might be able to handle. Another reason it might be bad is if you download something/anything it could be a virus and screw your phone up. But a good thing with it is that you can completely customize your phone with it, like whatever you want to change in you phone you can change it. Oh and before you do it call apple to make sure your warranty is over because if you jailbreak it and you take it to get fixed they won't do it because you jailbroke it