I don't normally progress beyond 'Normal' difficulty-- aside from the fact that I'm a mediocre gamer, I don't like it when the enemy gets health/attack/wjatever bonuses (or when I get penalties). If I really like a game that I play again and again, I might progress to a higher difficulty if the only bonuses the AI gets are accuracy, reaction speed or intelligence, in other words, simulating more skilled opponents instead of merely opponents who are harder to kill.

The highest difficulty I've gotten to was Expert on Condition Zero, back when I was playing daily with other human players, but once I stopped, everything sort of just slipped away. Now I can play maybe on Normal or Hard only.