The stock flightplanner in MSFS is lacking in a lot of areas, and is relatively quite clunky to anyone that is used to EFB (Electronic Flight Bag) software. Dealing with user waypoints in particular seems to be a real hassle.
I use AvPlan for RL flightplanning, which exports GPX files. I understand Foreflight exports XMLs? It would be really nice if we could import these into the sim.