I used them and was very pleased with the result. Don't expect them to make a crappy essay good, however. I spent a ton of time on my essay before I sent it in. The best improvements they made were related to the sentence structure and organization of my essay. It was a lot "tighter", more direct and had a better feel to it. Of course, I didn't incorporate every change they made, but it was nice to get some feedback besides my wife and parents.
Note: this just refers to their essay editing service, which cost me around a hundred bucks. If you want them to help you write the actual essay, they have a more expensive program, but I don;t know how that goes.