Heskey is a very strong player and he is a very hard worker. There aren't many strikers who will go for the balls Heskey often sees going in his 'direction'. He just never gives up, that's what I like about him. His weakness is, as said so many times already, his goalscoring, but if Emile keeps defenders busy, other players get more space, so it's easier for them to score.
To be honest, I would prefer a more technical player next to Owen, so this striker can hold the ball and gives an intelligent pass to Owen, and we can see more nice combinations, but I don't think we have such a player at the moment. I believe Baros can become a good player, but I'm not convinced about his technique and his game-insight, yet. I haven't seen Mellor playing enough to judge about him.
That's why I still prefer an Owen-Heskey partnership in attach, although I know the attack can be much more dangerous when Owen has another kind of striker next to him.