Well...I'm actually smack bang in the middle of my work experience right now, and I'm at Sydney Children's Hospital. I have to say, it's really interesting, and I think it's one of the most rewarding experiences of my life- because now I've decided that I definately want to be a doctor now.
So when you do choose work experience, I think that's a good idea to find an organisation that's pretty big, and actually has a work experience program that is really good, i.e. hospitals, law firms, etc. That's if you're ambitious though ^^. It also requires you to either be really early, really impressive, or have a contact in the organisation.
Good luck!