Is the Panasonic 12-35 f2.8 any good for real estate photography?

Unfortunately, I think all of you are missing the point. Using a ultra-wide angle will obviously show more of the indoor space in one shot but will also exagerate the dimensions of the room in question.  If you're an "honest" real estate agent, you'll want to use more shots with a "normal " perspective (i.e. 50mm equivalent at full-frame, 25mm m4/3) instead of only one 12mm shot of the same room; the real dimensions will be better rendered for the potential buyer.