The use of 3G internet in the cell phone brings incredible advantages but may also be accompanied by a high bill at the end of the month. There are already several unlimited 3G data plans with no deductible in which you can use as much as you want and will not pay more expensive than the combined. Other plans are unlimited but have a franchise of data, and the speed of the connection is reduced to match the moment the franchise is reached.
If your plan is unlimited ( internet without franchise ) you have no obligation to control how many MB or GBytes you are using every month but you may want to know to get an idea of the internet consumption of each application.
If your plan is not totally unlimited (since 3G internet without a franchise is very expensive yet) then yes it is good to keep an eye on the franchise consumption, especially in the plans that charge for KB that exceed the franchise. In these cases it is good to leave the 3G to use only in the moments when it is really essential.
There are several app to measure consumption of 3G internet but several have side effects like leave the phone slow or even spend your internet to download the advertisements that show. A simple application that does what it promises is NetCounter. No flaws or too many unnecessary setups.
NetCounter measures daily, weekly, total, last 7 days, monthly and several others, both 3G internet and Wi-Fi! It shows the amount of KB your phone sent or received, and allows you to set alarms for each of these periods.
NetCounter can warn you, for example, when you are close to reaching the limit of your franchise, to stay alert and avoid overtaking!
Visit the Android Market and download the 3G usage meter, NetCounter . Then leave a comment here saying what you liked the application.