I haven't used an essay writing service but I've seen other people post their experiences with these services before. Their reactions have generally been pretty mixed - most say that while the service usually did a good job correcting grammar and making the essay flow better, they felt that the service also edited away their personal touch and made the essay seem rather sterile.
I'd recommend getting every one of your family members and all your friends to read your essay and offer feedback. Of course you can't make changes based on everything they say, but they'll give you a good idea about what's good and what needs improvement. And since they know you, they won't change your essay to make it seem like someone else wrote it - it will still be your essay, just better.
There's also usually a thread that pops up in June or so with SDN'ers who've been through the process offering to critique PS's. You can also post a thread requesting readers of your PS, usually you'll get several people offering to do so.
Good luck!
