Not much I can say about Crysis Warhead that I didn't say in the Crysis post. The campaign is pretty much the same thing as the original game with few improvements like couple new weapons and better pacing. It even takes place in the same place and time.
I finished the game on the highest, Delta difficulty this time. I hardly noticed any difference to Hard. While the enemy AI is smart enough for finding you, it doesn't have a very fast reaction time. Even on Delta you can take out a whole squad of soldiers before they start putting up a proper fight when you get the jump on them.
Quite the immersive experience the game was, though, I must say. After loading a new level, I thought I'd only play just a little bit more, but then always found myself at the loading screen for the next one.