For my essay I wrote what they asked - my theme was basically "tell us things we need to know".... I wrote 1500 words, and under about 4 sub-headings. Basically 4 mini-essays, simply about things that when I read my application in full I felt deserved more depth.
Like everyone says, it's completely individual and I suspect that's the point. How you answer the questions says as much as what you write!
I got invited to interview, and mine is on Tuesday (Eeek!).