We use this at the office and it works pretty well. It's not exactly the same as writing on a real whiteboard, but having an entire wall for drawing database diagrams is very useful. Our wall is like most walls, I guess, in that it is not perfectly flat like a whiteboard. The small divots and bumps make it nearly impossible to clean (especially if the writing has been left there for a while).
The cleaning is the biggest thing. I've seen this in a few places and there are always red and blue tints left over from erasing. Old writing tends to be a bit hard to take off without some kind of liquid spray.
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.
