I think it is important to use your PS to show how your background makes you a good candidate for law school. Discussing what is unique or interesting about you can reveal this, without specifically saying "I want to go to law school because..."
Try to make your PS interesting and, of course, personal. The statement should have something they can remember about you, and show them that you can write intelligently without using your PS to goof off and make them laugh. That's about all I know about personal statements.