You can control your usage toggling on the
button. It will show you a panel in what you will see on one side some information about your connections along the current month and session and on the other side a pair of bar graphs with your usage of 3G and GPRS connections.The information you can get here is:
The month whose information is showed
The transferred data in the month. Using GPRS and 3G, and se sum of both.
The same data of the current session
There is also another tab to watch the same information of the last month.
In the usage tab of preferences window, you can choose to receive a notification if a threshold is reached, so you can control your consumption.