Very few people have unlimited data for their iPhone, so a tool like Data Usage is very useful, showing you quickly how much you've used and how close you are to your limit.
Data Usage provides only what it promises to show you. There are two readouts on the screen--one shows how much data you have used and the other shows your limit. You can change either number with the tap of a button and the graphic at the top will show you visually how close you are to your limit. In tests, the meter proved to be fairly accurate, though never the same as the numbers given by the carrier we were using (AT&T). If you need a general overview of your data usage, however, this will get you there fairly easily.
Data Usage is a basic app that serves an important purpose. Instead of having to log in to your carrier Web site or app to check your usage, you can quickly and easily see how much you have used and how much you have left with this app. The ad at the top of the screen is static and therefore not frustrating at all, the graphic changes as your usage increases, and the numbers are very close to accurate when compared to what AT&T has recorded for our account. Better yet, it's free, which makes it an easy app to recommend.