DataMan tracks your data usage, sends alerts for data caps
Unless you're on the AT&T unlimited data plan, you may be sweating those data caps on your iPhone. If you go over a certain amount of data usage on your iOS device, you'll wind up with overages. Couple that with the fact that our iDevices appear to upload info in the wee hours of the night and consumers wind up wondering just how much data they are using on a daily basis. Yes, you can drill down into Settings > General > Usage to see your data, or use the AT&T myWireless app, but those are slightly inconvenient and don't provide alerts or detailed info. Enter DataMan, which features data tracking and push notifications to let you know when you're getting close or going over your data plan allowances. Does it work? Absolutely. I tested the app for a few weeks and found it quite simple to use.
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Interface
You have three view options in DataMan: Current (a summary look at your data usage), History (a date-picker allows you to dial back in time) and Settings. The interface is comprised of standard user interface elements from iOS, although the back button in a few places is unlabeled and potentially confusing. Otherwise, the app is clear and to the point — it is not a fancy, over-designed app.
Using the app
In the Current view you'll see your cellular and Wi-Fi data for the day, week or month. Listed in megabytes and as a percentage of your budget for that time period, you can tap each list item and drill down to see the download and upload usage in MB. There's a little redundancy when you drill down, as the secondary screens will show Wi-Fi and cellular usage, but that's more of a time saver. Think of the Current view as your dashboard.
History allows you to dial back to a start and end date of your choosing to see total data usage. It's a standard time picker and shows the same view as the Current views when you drill down to a specific time period. In this case, it'll show you Wi-Fi and cellular usage for the time period requested.
Settings will be where you want to go when you first use the app. Here you set the billing date, very important if you want to avoid overages on data use. Then you'll want to set your data allowance numbers by the day, week and month. It would be cool if the app automatically calculated the spread for the days/weeks after inputting a monthly number. As it is, you could set 400 MB in daily but have 200 MB for monthly — somewhat less than useful, I think. Still, if you are looking to avoid overages, just leave the defaults and set monthly to whatever your plan allows.
Lastly you'll want to set the alert thresholds, which will send push notification warnings when you have approached a percentage of your allotted data use for the day, week or month. This is very handy and worked fine for me.
Precise tracking is default to On, and will log usage every 10 minutes. The only downside to leaving this on might be battery, but I found Geotagging (off by default) much more taxing on my battery. If you've used a backgrounded GPS app you know what I mean.
Conclusion
If you need to track your data use or set alerts for when you're approaching usage limits (also handy when traveling abroad), DataMan is exactly what you need. For $1.99 it does what you need for a decent price. There's a lite version if you want to try it out, but this is one of those apps you should just get if you are sweating data usage limits.