I agree with QTkitty, I think you'll get better results by having as many *different* people as possible read it and give you comments - friends, family, professors, veterinarians. They all know something slightly different about you, and you definitely want opinions of people who don't already know everything about your application. I was pretty surprised at the variation in things that different people picked up on. Get their comments and then revise the essay yourself, rather than asking people to make revisions, and it will remain in your words and true to your character.
Plus, the "professional service" is probably just some out-of-work English major. People whose writing is really $200 better than mine have better things to be doing with their time and skills than revising application essays. 🙂
Pretty hard to judge how much my personal statement affected my acceptance - the interviewer who had seen my file didn't draw out anything specific from my statement during our conversation, so I honestly don't even know for sure that he read it...