Seconded. I don't even play single missions anymore without editing them with the editor. I now wonder how SF2 managed without it.
IMO, it's biggest flaw is that I take probably more time setting up the missions than I do flying them - maybe a copy / paste feature or units templates would speed this up but probably won't ever happen.
Being able to save individual missions from Campaigns to then use in the editor is a really great tool too. If I remember correctly, it'll even remember the that campaign mission's "borders", meaning that Soviet forces might be deep into West Germany (or vice versa) depending on how your campaign was going at the time.