I think almost everyone should do one if possible. In most cases, it helps than hurts.
Reasons why you should do an away:
(1) Just to figure out what kind of program you want to end up at. Do you like urban vs rural? Do you like academic vs. academic?
(2) Signal your interest in particular geographical region.
(3) Show programs that you are a competitive applicant. Away rotation spots are somewhat limited at more competitive places. So, landing one shows other programs of similar caliber that you are someone they should consider.
(4) Few programs pull very heavily from away rotator pool. There are advantages to being a "known stock."
Pitfalls of away to avoid:
(1) I think even mediocre performance at an away is better than no away at all. But, subpar performance can hurt you. So, I strongly recommend that you get your feet wet at your home program before an away. (something I failed to do.)
(2) Make sure you are interested in the geographical locale. (I know it sounds common sense. But, this is NOT something I thought about when I was applying for aways. I did my away in an region I was not really interested in just because I liked this one particular program alot. I don't recommend others do what I did.)
(3) Make sure you do your EM rotations early enough to get SLORs in ON TIME (sooner the better but definitely BEFORE MSPE comes out which is going to be mid-October for current M3s).
How do you get an away:
(1) Look at the program's website. They will tell you whether they are on VSAS or not.
(2) Get your stuff in ASAP. VSAS is a bitch. It's pretty annoying to get all your forms converted to pdf and get more vaccinations that you don't really need. But, you gotta do what you gotta do. Some immunizations and tests (titers, ppd, etc...) takes TIME. So, get on it.
(3) Be nice to clerkship coordinator and ask her specifically for advice if you can get a hold of her. This will get you far.
(4) Don't over-apply. Most people get the aways that they want. So, don't unnecessarily hold up spots you are not going to take. You can always accept a spot and drop it later. But, it's just a bad form. I don't think they'll remember you and hold a grudge or anything... Still, remember that you might be applying to the same program that you are dropping for an away. So, always be courteous and prompt as possible. Use common sense.
Few other things I wish I had known as a M3:
(1) Programs will interview you even if you are only scheduled to do an away at their institution. Let's say you are really, super interested in program A. You couldn't get an away schedule in the high-volume months, but still signed up do an away in February. The chances are that Program A is going to offer you an interview even before you do your away. This is probably not universally true... but true enough.
(2) There are different types of EM rotations available for aways: ultrasound, tox, wilderness, and etc.
Alright, PM me if you have more specific questions.