When you choose someone to look over your admission essay, you should choose someone who will not be afraid to tell you like it is. In other words, don't just show it to your mom or your significant other. When you show it to someone who isn't afraid to give you criticisms then you are doing yourself a giant favor. Remember, those who review it when you turn it in will be looking not only for the good, but the bad as well. If your statement is not among the good, then it will get weeded out with the rest of the bad.
Another important role of the PS is that it allows you to explain, although briefly, some information that may not be accurately represented by your transcript or Curriculum Vitae. For example, your GPA might be low because your school places a cap on grades given to students belonging to your major. Another example would be explaining the connection between your undergraduate major and your chosen graduate major, which might seem incongruous at first inspection. For these reasons alone, it is extremely important that you follow a guideline in composing your own PS.