The balance isn't great, and the idea is even worse (oh, the horror! I thought Heroes 6 would develop the events after Heroes 5, but no! They rewound the story back centuries – they might as well have remembered the Stone Age...). I personally didn't understand the magic (or its development) – although it has a style reminiscent of Heroes 5. I haven't tested all the cards, so I'll stay silent about that. And there are really few units and factions (I like having a huge choice of everything).
Of course, I'm not saying that the game won't get better or worse over time. And I agree that the game will find/has found its fans.
But the developers finally need to think about the customers – because you really pay for an idea, a game (in the full sense of the word – a GAME), and not just a beautiful box and a beautiful GUI (interface).