iOS 4 brought multitasking, folders, unified inbox, threaded email conversations and note syncing to the iPhone. For these reasons, some people think that jailbreaking a device running iOS 4 is now pointless, considering that two of the biggest features of jailbreak (multitasking and folders) are now developed by Apple itself.
Did I jailbreak my iOS 4 iPhone 3GS? Yes. Do I think is still worth it? Yes.