As others have mentioned, I feel a new game plus feature of some sort is really the only way that unlockable difficulties are ever acceptable. While certainly possible, I can't imagine trying to beat Mass Effect on Insanity with a level one character on my first play through of the game. It's certainly possible, but that would strike me as an act of masochism. I do think that the increased difficulty should be well worth it if you have to unlock it, though.