FPV
Although it can be useful i almost never use it.
Dont use it in Airspeed Unreliable. It will show on the display but pitot/static information is fed into it somehow so it cant be garanteed to show correctly!
The times i would use it are on a (night) NPA in gusty conditions, after you switch off the F/D's the FPV symbol doubles in size. You can determine your vertical path (relative to the horizon) as well as drift (the FPV symbol below the vertical bar is 10 deg, end of the horizontal bar is 20 deg). The FPV is very sensitive therefor it helps in detecting windshear.
Most pilots I fly with dislike it. Same goes for VSD but thats a different topic.