I'm still having trouble getting something I really like with water stamping. Funny you post this this thread, because I only just gave it another try the other day...and ended up stamping in ink and embossing. And I saw some gorgeous cards posted when it was the technique challenge one week.
I love magic cards!! My tip for stamping on acetate - use a stamp positioner, then your stamp doesn't slip on the smooth surface.
My problem with the split negative, as described in the post above, is that I don't have a white EP that's the same "white" as my card - and I've tried several. So I don't like the look I get.