I have the HD 2 now more than 3 months and Stackz works fine. The larger screen is really helpful. You can't use the hardware buttons any more, that's the only thing I miss. But even when the buttons take away a part of it, the screen is bigger than on my old device. Ah, and sometimes I miss the stylus, when I want to move the lines in List View, but I don't need that very often, and you can get used to that by practise.
To say something more positive, Stackz works very fast on the HD 2. Much faster than on my old device. That's really fun. To scroll with the fingertips is very easy if you get used to it. Maybe it would be smart, if you could get to the next card by gesture in Test Modus, like similar functions in the calendar or other applications running on WM 6.5.