I did an away at Columbia, ranked it first, got recruitment calls from the PD and recommendations from attendings that month, didn't get in.
I think aways are useful. I'm sure I was ranked more highly there than I would have been had I not done an elective at NYPH but apparently it's not always a sure thing. Do one if your first choice is ultra-competitive (Columbia, Stanford, MGH, UCSF); it'll certainly help and can't hurt unless you don't do your job well. I'm happy where I'm going though (Pitt) and did not do an away there. Match can never totally be understood. Good luck.