Mine was very dry--just describing past experiences that demonstrate my competence for graduate level work (research, employment, grant awards, publications, academics) and why I was applyig to that particular program, citing some mutual research interests I had with faculty. I didn't include any personal life fluff at all, nor described some road to Damascus event that lead to my interest in the field. I didn't present an idea for a dissertation topic or what I wanted to study in much particular detail--my fear is that would have been too narrow in scope, as it may only be relevant to a small number of PIs at any given institution.
In short, I described why they should want me and what credientials I had to prove it, and why I wanted them--no fluff. It was about one single-spaced page in length. That's it, and it seemed to work for me.