I play on different difficulty levels depending on what strategy I'm using, i.e. how I expect to win the game.
If I'm using a custom sovereign and going for a conquest/quest victory, I play Insane (ever since I learned it existed)
If I want to design and deploy troops, I play on Ridiculous (and lose 50% of the time if I play with a "No champions in enemy dominion" rule)
If I want to experience the full range of the game - diplomacy, alliances, economic tug of war, I play on Expert.
When my wife wants to watch a game, and be the back seat driver, I play a game on one level below the highest, without placing limits on what I can use, because I want her to enjoy it, and she enjoys a smooth victory. That has happened three times...
I have not really played standard race/sovereigns since I won with Altar on Hard in 0.912 but I think I'm about to. I will try Relias/Altar on Insane.
[Edit: Relias is very, very, VERY poorly designed. No life magic, and adventurer as a profession? Please. He is nonviable on Insane, unless his starting hero has life, which did not happen in five re-rolls. I think I will try a custom sovereign with Altar, or Procipinee/Pariden)