Although I feel like shadowing a physician is a great way to start learning about medicine, I don't think that it will really show an adcom that you've taken a strong initiative to explore the field. Most people at some point will have done it, so it won't really differentiate you from the next candidate. I also think there is a BIG difference between shadowing and volunteering. When volunteering you are fulfilling a need of the organization. If you have the time, I would look into other clinical opportunities. There are a lot out there.
1) Clinical research at a hospital/university
2) Children's playtime/playroom at a hospital
3) Convalescent hospital (not as much medicine, but a lot of hands-on time with sick and elderly patients)
4) Volunteer in medical clinic for underserved population (if you live in big city or near a poor community)
5) Health peer counselor at school (some have general ones, some have ones related specifically to AIDS/HIV)
... many more.
When shadowing ask the physicians or staff if there are volunteering activities available or look for a need to fill. There are a lot of opps out there, you just need to look. Doing so will show an adcom that you have the initiative. Straight from the mouth of someone who reads at Stanford, they said "I really value someone who identifies an area of need and finds a way of helping to fill that need. Honor societies and awards impress me far less." Again, shadowing is great, but if you want your clinical experience to be a strong part of your application, my feeling is you need more. That being said, if you are volunteering in other important venues (education, social services) that should also help your application.