My first thought was: "who would use such a service?"
I don't have much of a "medical history" to keep track of. I'm not eating any regular medicine and never had, and in any case it is not something I want to try to keep track of myself.
I trust my physician and would want him to figure out what medicine I need, if any. I want him to keep track of my medical history. He is the expert, that is why I go to him. I don't want to second guess experts in a field where I have very little knowledge.
So really, what's the point in using Google Health?