Do most people decide to do away rotations and if so, why exactly? Is it just to experience a new hospital setting or anything else? I think i heard that if you want to match in a particular program it is good to try to do a rotation at the school and impress faculty over there in the process - if so, how effective is this usually? Also, does anyone know if you can do your core clerkships away and if there is a limit to the number you can do?
Away rotations can been good for gaining an experience that you otherwise would not be able to achieve at your home institution.
I always caution against doing an away rotation just to "impress" the residency program director because you want to be there. If you are a strong candidate for a program (good grades and good board scores), you don't want to take a chance on ruining your chances of a match. Impress them during your interview and leave it at that.
On the other hand, some of the more competitive places in some of the more competitive residencies like neurosurgery, will not interview applicants that have not done an audition rotation. If this is the case, schedule your "audition" early. These program generally have some kind of approval process so get that information and get your application done early in your fourth year. Do beware that these types of programs will not approve your application for rotation unless you have the grades/board scores to be a competitive candidate for their program.
Doing an audition rotation can also backfire if you are a strong candidate but "screw-up" during that rotation. There are many definitions of "screwing up" but some residency applicants have been able to "tick off" an attending or two and ruin their chances of matching. You may get the interview but you won't be ranked no matter what how good you looked on paper because they don't think that you will fit in or they just don't like you.
Doing an away rotation can solidify or change your mind about a particular program too. Again, many applicants will do an audition rotation, do an excellent job and enhance their chances of matching if they were competitive to begin with. Doing an audition rotation is not going to overcome major deficiences but can put you in a more positive light because you are a "known entity".
The bottom line with an away rotation is that you weigh your pros and cons. Doing an away rotation at a very strong program can solidify your educational experience and bring a dimension that you previously had not experienced. In that manner, you can only benefit. If you are a weak candidate (grades and boards), you can only help your cause if you do an audition rotation and impress everyone. If they like you and you do a good job, they will rank you.
On the other hand, if you are a strong applicant, you are going to get the interview so do your impressing there. Go back for a "second-look" if you think you need more exposure and do your audition at another "reach-program" where you need to impress.