I did a bit of agency work one summer, it was fairly pleasant. I had 40 hours a week in one place sitting around drinking tea with the residents and being fed for £6.50 an hour. On the days off from that I got long days at the hospital (14 hours at generally the same rate) the trick there was to obviously be a medical student temping over the summer then the nursing staff seemed to expect you to be hopeless and just ask you to do obs rounds and make tea. Occasionally school nurse stuff where I got £14 an hour to play board games with kids who were slightly tired and eat a constant stream of sandwiches delivered by beautiful spanish girls.
Good stuff for a summer job, I could work every day and rake in the cash, but I wouldn't do it in term time. I certainly wouldn't do it in a hospital where I might study, I saw one chap mid ward round be told by the consultant to go and do some HCA stuff as 'he works here'.