Why do you need a service to review your PS? Ask your pre-health advisor/your teachers and friends. If they think it needs work then it should be sufficient. Having shown my PS to my research mentor (staff at medical school), english professor, friend and brother I felt pretty good. People can argue that these people don't know what med school admissions are looking for but on the contrary, if things don't make sense to them then you should rewrite your PS. I feel like these services are gimmicks to steal your money. I am sure they do a good job, but I do not think you need to use them.
You university should also have a writing center, atleast mine did.
Also the essay is supposed to define who you are. You only want people to revise it for you not have someone rewrite it. I am sure medical school admissions can pick out those that just don't seem to match with other writing promts you have on primary application (such as activities descriptions)