While I didn't do a rotation at any of the Harvard affiliates, I can tell you from first hand experience (good friends) that if you aren't qualified on paper, it's a waste of time. They won't offer you an interview just becuase you rotated there. However, that being said, if you're decent on paper (AOA and/or 220's+, and/or significant research) then it's a chance to work with a good attending, score a solid LOR, and use it to your benefit. But, for all those things to happen, it's difficult. your chances of working with an attending whose letter actually makes an impact are smaller than you think. Also, you have to see what kind of person you are. Do you think your clinical skills/knowledge/work ethic can impress people? I didn't know about mine either, but I did a rotation at one of the aforementioned places (that tends to not offer courtesty interviews) and was able to get an interview.
So, bototm line is--I think it definitely helps, but you should be a decent applicant. If you are not AOA, step 1/2 below 225, I would steer away from it. Obviously, there's no hard and fast rule. But, my friends who had these stats were not able to get interviews at those places. And, I also have friends who had 250's, AOA who did not get interviews there (they didn't do aways). I also am from a smaller school, so I think if you're a good applicant on paper, and coming from a smaller school, and THINK you can impress these guys...do a month, and work your arse off. I think it's worth it if it's truly where you would like to end up at. plus, it will be a chance to check out one of the elitist programs in the country. And, you will learn. I would recommend, like another poster said, doing a month you are comfortable with. An ICU/CCU month is always nice (despite working hard), because you aren't expected to know too much, and you will learn a lot. A ward sub-I month will expect you to know A LOT. Do a month of whatever you're gonna do at your home school so you are well prepped. Just my 0.0002c. Good luck.