Not very useful unless you buy extra features. – DataMan - Data Usage Widget Review

The thing that looks most useful in the sample screens shown in the App Store is the hourly tracking. But that is an extra cost feature. Without that it does not provide anything more useful than the cellular data stats built into IOS. The developer suggested I look into the widgets, and I agree that the "stopwatch" widget is useful for measuring the usage of a specific app. I upgraded my rating from 1 to 2 stars because of this. If I could I would give it 2.5 stars, but I cannot go to three stars because I feel the presentation of what the app does without extra in-app purchases is deceptive.
Review by tdahlia on DataMan - Data Usage Widget.