Haven't tried legendary yet but heroic wasn't bad.
This is definitely not as difficult a game as Halo 2. Feels much more in line with H3/Reach to me. That's all assuming that the jump from heroic--> legendary is similar to what it was in previous games.
Definitely don't understand the reviews that called normal the new heroic and said that heroic got bumped up a notch in difficulty... that's not true at all. More like reviewers died a lot because there's a learning curve on how to use new weapons and fight new enemy types.