I think the big problem is that plaster walls just aren't flat enough to prevent ink hiding in the divots and creases. So you can wipe over the surface but never get the little spots out of those imperfections.
We've painted a whiteboard-sized area of our office with this stuff. There's no border to tell you where the whiteboard paint finishes. As you'd expect, the ink is no longer confined to the bit that's wipe-clean. Otherwise (with the spray proviso above) it's been very useful, and we can always extend it with another pot later.
Another alternative is those statically charged pieces of "whiteboard paper" - Whiteyboard is mentioned below, and there's also http://www.magicwhiteboard.co.uk/ - although they don't stay up tidily for much longer than a couple of weeks. I think they're only really meant to last e.g. a day-long meeting.