I bought it myself immediately when it was released, and it's great.
I think it's the novelty of the idea, coupled with the graphic nudity. The stop-and-go idea causes extreme sense of desperation, which Rachael so vivdly shows. Plus, some of us guys are intrigued by the female plumbing system and like to see the peeing up close
Sales-wise, it's an interesting question whether or not you can make any more of these before the idea gets old. Or perhaps some new idea along the same lines? You were talking about multi-model shoots in the previous thread. Think of this - TWO models, each with a tiny cup, competing with who can fill the most cups without spilling, and without breaking down, with some prize for the winner to make it real. They should pee one after the other, not at the same time, so the camera could zoom in on each one individually (and us wierdo guys can compare the different female anatomies). Now that could beat your sales record again