The iPhone's built in calendar app can described as functional at best. Pocket Informant is a calendar and to do list manager which enables you to focus on what needs to be done. A selection of customisable views means that the app can be used to focus on your upcoming events in a variety of ways - and it is easy to switch between views, such as week or month mode.
Your to do list can be integrated into your calendar so both sets of information can be viewed simultaneously. Data is kept separate from the iPhone's calendar which means that it can be used to keep different types of appointment, but one of the key advantages comes from the ability to synchronise data with other calendars. Synchronisation is available for Google Calendar and Toodledo, so Pocket Informant can easily be used as your manage appointment management tool rather than just as a supplement to your current tool.
The use of timebars to represent meetings and appointments you have set up makes it possible to view your free time at a glance, and a number of filtering options means that you can quickly track down particular calendar entries without endless searching.
The paid for version of the program adds a number of useful features including support for push alarms and the option to synchronise custom date ranges. Synchronisation with Outlook is also available.