I've used the services of Kaplan admissions consulting and have actually found them helpful beyond what SDN and school writing centers can offer.
First of all, most school writing centers do help you write a more succinct essay, perhaps, but they usually lack any sort of knowledge of the medical school admissions process. They don't know what med schools are looking for. When you pay for a service through Kaplan, you usually get someone who has been on an admissions team for a while at one or more medical schools.
Second, while SDN does have a lot of great ideas out there, you're left to your own devices. Some argue that Kaplan produces some type of pre-med prototype. To this I respond, So what? If it works. SDN members, while often well intentioned, tend to have varying views on ways to approach essays.
Sure, you can get great advice on the very general stuff anywhere-- Study hard on the MCAT, apply early, have good grades, etc. But in order to go beyond that, I have found consulting quite helpful.
Plus, in the scheme of things, as much as you're paying for college + test prep + interview flight / lodging / food, why would you want to skimp on something that could help you get where you want to go? It's a drop in the bucket in the end and I've found it immensely helpful.
