Hi, I have Flight1 AFX installed since my FS9 days but I have to admit that the only feature I have used in FSX has been the Flatten Polygons tool to create new AFCADS to resolve elevation issues in a number of airports. With this tool it is necessary to reload FSX to see the effect.
I did make some modifications to FS9 airports many years ago using AFX and, using the FS Preview function with FS9 running, I was able to see the changes as I made them 'live', if I remember correctly, so I assume the same would apply to FSX.
It is quite a powerful utility though and comes with a good manual and I can recommend it if you are keen to modify FSX airports.
Hope this helps,
Bill