Its functionality in flight planning is pretty limited (lines from HERE to THERE but little else)
You start off by inputting your origin and destination (via the keypad usually) but from there you can "rubber band" the route to suit. Via existing waypoints/navaids/VRPs, or via user-defined waypoints. Although I found the catchment area of the magenta line quite small - it's very hard to get your finger in exactly the right spot so that it will rubber band your route instead of scrolling the map.
What I'm missing still is vertical navigation. In the latest update they've added a vertical profile which supposedly shows terrain and airspace ahead (I have not flown with it since that update - can't comment on it) but that's still not the same as proper vertical navigation, taking into account climb and descent profiles and possibly even the cloudbase.
What I would like to see, in addition to the rubber band technique, is a user-editable plog where you can input waypoints, altitudes etc. via a keypad.
Still, if used intelligently, it's a great tool.