Heroes definitely adds a new dimension to the game. Played five games tonight and the expansion either played a key role in some way or seemed to have no effect whatsoever in each game.
I made a new commander type hero with 'lead by example trait' and my buddy made a mage type hero with 'ball lightning.' Lead by example sounded like a really cool leader card to me, but in the five games, it did not once get triggered by a scout card. Just bad luck I guess, but I never had an opportunity where I drew a scout card that made a better move for me than the other available cards. My buddy knocked out maybe three extra figs total in the five games due to his ball lightning. Ok, so it sounds like the heroes didn't do much, but it changed the way we played.
In one game I lost my commander at the very end to a mounted charge. I won the game by two flags, but I still felt that I had lost especially since I didn't have a treasure token to pay ransom and my commander lost two hit points. In another game, I set up a careful ambush and used 'greater portal' to pull his mage into the ambush with the hopes of killing him. Turns out that he almost fought his way out of that and just barely lost the game by one flag.
Point is that the heroe's life or death really felt personal to us playing the game, and altered how we made our plays. We've really enjoyed the expansion. Thanks Richard and everybody else at FFG.