I think I prefer the special characters over the curse word in most instances. The only time when I might change my mind on that is if I have to stop and think about what they were trying to say, which ruins the flow of the dialogue a bit.
For instance, if I see "What the %#@&???" then I can generally assume the character was surprised by something and said, "What the fuck/hell???"
If I see, "Jason was running over to the $%&@ grocery store." then it's not quite as easy to figure out and can slow down the dialogue a bit for me.