Played my first games against the DA last night.  My impression was that, outside of the swap ability, he's mostly harmless.  But I do have some issues with him:

1.  He's too damned small.  He's impossible to heal or shield in a big fight because you can never pick him out.  I swear he's only slightly bigger than a minotaur.  Picking him out to attack is tough as well.  If you're fighting against a character with a bigger model, like Rook, Sedna, QoT, UB or Oak it is possible for a DA to join the fight and you not even notice.  Even if you did notice, there's no way to click on him if he's squashed in there with other enemy DGs.

2.  Swap effect is too subtle.  Swap is nasty, I admit.  But my biggest problem was that it was very difficult to tell that you had been swapped.  In big fights, when all the DGs models are merged, you can get swapped out and it could take a few seconds just to locate where your toon went!  This could be fixed by: 1.  autocentering your DG when you're swapped.  2.  Adding a more visible graphic when you're swapped.  

I think DA becomes more powerful the more players involved because he can take advantage of more confusion.  A 1v1 fight with DA is not tough, 2v2 is OK, but 3v3 or 4v4 is unmanageable.  Large fights are already difficult to manage and the chaos that DA injects is tough to counter, especially because he's so difficult to find and target himself.