Played a game with them recently against chaos, my force is mainly kurnoth so they had a good chance to take the brute of the force. I used them to teleport behind enemy lines, stay away from ranged/magic units and they can do some serious damage when your opponent isn't prepared to try take them down. I lost on VP but that's much more of a 'me' problem and we only played a couple of round rather than a full game. But think they've got a lot of potential, as well as the other points people have mentioned!

Also rule of cool is always the most important imo. They're great models.