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Pros
Pro You can also track phone usage and the number of times you unlock your phone
Pro Provides different views for the statistics
Pro Free version's ads are not intrusive and don't pop-up
Pro Allows you to choose the apps to track or ignore
Pro Highly customizable persistent notifications
There is a load of information you can display in persistent notifications, from global phone usage, a remaining time notification for each app and even a most used apps notification you can use to switch app.
Cons
Con The pro version is rather costly
Con Not open source
Con Sometimes loses database
Database (history) sometimes gets lost, either by corrupted database or simply emptying it.