I was a volunteer EMT/firefighter for two years, professional EMT for a year, and worked in the ER for 6 months. I'm sure that ADCOMS looked upon that experience quite favorably because it showed a high level of committment to health care in general.
However, I did that because I wanted to be an EMT. Your post is vague as to what position you'd have, but if you will actually be working on an ambulance you'd better be doing it for the right reasons. It is very difficult both physically and mentally. Your back will take a beating, and when someone calls 911 because their premie isn't breathing they expect you to take care of it. It's a great experience if you really want to learn some patient care skills long before clinic starts. It can also be a lot of fun. Just make sure you're not doing it solely for recs and your app, because the patients, your back, and your sanity deserve better.