One should never use user waypoints in a GPS, due to the potential for data entry errors.
I Use them all the time
There are many reasons you may use a user created waypoint. Obviously if you have a VOR NDB Intersection etc use those to load into your GPS but sometimes you have to create your own.
The main point with user created waypoints is to be aware that there is room for error and never trust one item back it up! Ie in the simplest term is the waypoint in the right direction? Is the DME readout matching to the map distance?
Pilots often use user created waypoints for approaches based on NDB DME.
The GPS is more accurate and not prone to so many errors! used together to back one another up you can then fly with more accuracy using the GPS but backing up the GPS with NDB/ DME.
Flying a straight line across a ridge of high terrain you may have weather or strong winds where its not passenger friendly to plough over them. Use user created waypoints to avoid the hills but dont fly them with blind faith. Check!
Rely on one item in any nav piece of kit and you are asking for trouble.
I am sure flying an ILS you dont blindly trust the ILS but check spot heights on the chart against the glideslope.
I have known of pilots who have flown frozen needles blindly!
Even there its useful to have the GPS to confirm the localiser centre point matches the GPS inbound.
Flying back from INDIA in an old wreck of a Citation and having been assured the GPS had the whole world installed Europe was not there.We had to load every single point using lat long coordinates for the complete trip from Saudi Arabia (Yikes) but we double checked and confirmed each item and then checked againt the maps. We only got one out of about 60 points wrong and soon identified it as being wrong.
Pace