I was in for 6 years and I'd say it definately helped at most of my interviews. At the very least it gives you something to talk about. It seemed much more advantageous at a few specific interviews where the adcom was either a vet also or at least had some connection or interest in the military. But it's just the luck of the draw if you get someone that seems interested. At one school the dean actually had a son in the exact same field I served in, and to say that helped would be an understatement. He knew my name and background before I even introduced myself.
My advice would be to draw on your military experiences in the interview, but don't make too many of your answers come directly from your service. Mix it up with everything else you've done. You can tell if the interviewer has genuine curiosity so look for that before you start telling war stories. And depending on what you did in the military, tread lightly with controversial topics. Don't get political like some bitter vet but don't come off like a war monger either.
