You can see above how the app gives you access to your current balance and the amount of minutes, text and data you still have to use.
Meanwhile on the second screen you can see your recent charges. You might notice from my stats that two recent foreign trips meant serious data roaming charges. £3.74 in calls and almost £200 in data. Ouch! Time to change my tariff I think.
For you non-iPhone folk, don't worry, O2 says it'll be rolling out the app to other handsets soon. If you've downloaded the app tell us: were you surprised with your iPhone usage stats?