The app is free and is 4.8MB in size. I’ve been using it for about 24 hours now and it has used 21KB of data thus far. I haven’t noticed my battery dying faster (iOS 6.0.1 manages to do that on its own).
Once you install the app and set it up, you have to wait a few hours before seeing any results (obviously). What I like about the app is that it is visually appealing. Your app data usage results are colour coded and displayed in order of app that uses the most data, to least. You can go into each app and view your daily usage, weekly, and up to 30 days. The reason your battery won’t get eaten is that your data is stored in the cloud.
Some screenshots of my usage of over the last 24 hours:
As you can see, Instagram uses the most data (1.97MB), followed by secure Web access (1.95MB) and Tweetbot reaching 855KB. I used a total of 6.73MB in 24 hours. It will be interesting to see what my graphs look like in a week and in a months time.
If you want to track which app uses the most data, and help with cutting down your bills, download Onavo Count and see for yourself.