Personally....
I didn't get too much out of the ER. I found the doctors, nurses and techs for the most part too busy and too, um, detached to really take the time to answer my questions, let me watch, etc. And I'm no fly on the wall, but I was told many times that unless i wanted to go clean a bed or deliver a urine sample to the lab, me and my questions were pretty much in the way. I think it was A) the nature of the inner-city, large ER I worked in where the climate was rather overworked and understaffed, without a clear mission for volunteers and B) part of the nature of the ER biz, where I observed in my year, a need for practioners to detach oneself from the whirl of people. Plus, staff was ALWAYS changing and I was always having to introduce myself to new folks every other afternoon. It was hard to develop a professional, working relationship, and again, I'm no ogre, I swear! I'm sure many people will disagree, but it wasn't the most unique experience, and go into that knowing that.
I got more out of an indigent clinic situation, where I developed a one on one rapor with the staff and recurrent patients alike. Something to consider...