I did my externship at Hopkins this year and I was really worried it would be a bust bc so many of my faculty where I go to school told me not to do it and said that it would only hurt a strong applicant on paper.
Though I can never be certain, I think it definitely paid off for me bc as the previous poster said, it was more to see how well I fit in with the residents. Of course I worked my butt off, but no matter where you go you are going to work your butt off. Even though you can't possible be everywhere at all times (sounds doable, I know) the residents know you are there to learn about their program and the ones I worked with were very receptive to my questions and did their best to include me into the team.
I can't speak for all programs but I think if you are on the fence about a program and want to learn more about what life would be like there (aside from the 2 days you may have in town when you interview) you should do at least one to see what else is out there.
Good luck!
CT
