Clearly there are differing opinions on this, and I disagree with most of the previous posters. I didn't do it as a student and got my first choice. From the perspective of a resident I can simply say that, of the 6 or so rotators we have had at my residency in the last few years, we've only taken one or two. The others ranked us no. 1 (or so they told us), but the month was enough for most of the residents to say "no thanks" when asked if we wanted that person. I know of one case where the residents were adamant about not having a certain applicant because she was really weird during the month. She went somewhere else but we've come to discover that she was jut trying too hard and is actually a pretty cool person. That one month really did her in, though. I just haven't seen it pay off enough to take the risk of losing your top choice because people read you wrong.
Regarding 4th year rotations, I would do as little path as possible (2 months max) and focus on things you'll never get to do again like dermatology, maybe rheum, radiology, etc.