Fun isn't exactly the word i'd use to describe the game on it's hardest difficulty, but it was incredibly enjoyable. So much so, that it is now my favourite game.
Treyarchs other masterpiece, Quantum of Solace, falls down to second place.
I always say, if you don't play any single player game in the hardest difficulty first, the chances of you exploring every little detail of the game are very low.
I like to do easy just to enjoy the effort put into the game looking at scenary etc then if I find its fun I go to the hardest. COD5 beat in as recuirt.