I done the Jailbreak on my first iphone because that was before the app store was out, so jailbreaking meant you could get loads of cool games and stuff that non jailbroken ones could. Done it with my 3g one aswell, can't remember if the app store was available right when the 3g came out or whether it was only available after an update, but either way, you couldn't record video on it. So that was the main reason for jailbreaking that one.
Now I've got the iphone 4, and I don't see any reason to jailbreak really. I suppose you can get any app for free and I think there's something you can get to support flash which would be alright, but as JD said, I think jailbreaking the newer iphones is more hassle than it's worth.
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