I had my two older kids look at it, and that unscientific sample suggested that people may vary widely in a) their initial perceptions (one saw it CW, the other CCW), b) the fixity of their initial perceptions (one got two flips within a minute or two, I had to instruct the other).

Dunno if there has been any systematic study of this, but I'd sure be interested in how these characteristics are distributed, whether they are fixed or variable in a given individual, and so on.