The Free Space Cam Cydia tweak was recently released and what it allows you to do is keep track of vital statistics while using the Camera. This includes battery life and free storage that you have left for taking photos. When you are shooting video it also lets you know how much time you can shoot for until you use up all of your storage space.
In terms of the tweaks usefulness I would definitely put it on the lower side, if you are someone with limited storage who is always near their limit I can see how this would be useful, but other than that it probably won’t appeal to any other user group.
The tweak does have preferences found within the Settings app that allow you to enable/disable various vital statistics using toggles. Unfortunately they require a respring to take effect though which is cumbersome. One pro, however, is just like other onscreen elements of the camera app the statistics can be moved around by simply dragging them.
If you are interested in the Free Space Cam Cydia tweak you can find it in the BigBoss repo for $0.99.