For those of us suffering excessive data usage in iOS 6, there's good news. DataMan Pro, which had been previously pulled from the App Store, is back. What sets it apart from the regular version of DataMan is that it can track usage on a per-app basis.
It's currently on sale for $4.99 until Black Friday. If you're having data overages, it might well be worth it. I haven't had enough time with it yet to pass a verdict, but the interface seems a bit unfinished. For example, the "Back" buttons lack text.
Regardless, I remain optimistic. Per-app data usage statistics are desperately needed in iOS, and right now this is the only way to get them.
If you're on the fence, check out Adam Engst's overview on TidBITS.
Update: The developers have informed me that the back buttons have been intentionally left blank to accomodate the date text. I hope they can find a better solution, because even if it's an intentional design decision, it gives the impression of being half baked.