While not a burden for some, there are many iPhone users out there that are stuck using the minimum 200MB data plan. Keeping track of your data usage is a bit of a hassle, but like everything else in life, there's an app for that.
DataMan uses the built in data usage meter to easily tell you how much data you are using, whether it be with 3G or WiFi. Because it runs in the background, it can also inform you of when you get to certain data use thresholds (i.e. 50% data used, etc.). All you have to do to get the app working is to set up what day of the month your data usage starts and let it run.
The last feature, which is probably the neatest feature of all, is also the one that will irk many users. As you are using data, the app geotags your location so that you can see the places that you are online the most. Seeing your data use on a map is quite neat (although not all that useful), but it requires your location services to always be on. If you don't like the feature, you're out of luck because the geotagging isn't an option that you can just turn off. Talk about a serious battery drain. On a device that already isn't known for wonderful battery life, having something like a data tracker killing X% just by existing is a major bummer.
For a user who is perilously close to running out of data every month, an app like DataMan is an invaluable resource. With the inability to turn off location services though, you'll have to be willing to sacrifice a bunch of battery life for the service to work.