I just finished up a 4 year Navy HPSP commitment. I was actually in for 5 years, as the dirty little secret is that your intern year doesn't count for payback.
I did 6 months of hang time after internship waiting to class up for flight school in Pensacola, 6 months of FS training, and then three years as a flight surgeon in Norfolk, VA.
So I'm just starting my residency in EM. I chose a 1-3 program, so I am essentially repeating my intern year. I did consider doing a 2-4 program, and using that transitional year with the Navy to count. Thing is, your skills and knowledge deteriorate while being a flight surgeon. No kids, no elderly, and no chronic diseases means that any "real" medicine is supplanted with flight physical jargon, PRT pain and suffering, and general sickcall nonsense.
When I went through the interview trail, some programs were receptive to my military time and training, and others were decidedly not. There is another former Navy FS in my new EM class, but my program is very military friendly.
I hear that the Navy is now allowing some "straight-thru" EM residencies, and even some full time out service (FTOS) EM residencies in civilian land. That was one season, though, and has absolutely no bearing on next year, or for whenever you manage to get to the Match.
I don't regret my Navy time, as I would have missed out on some interesting experiences, but I definitely feel older compared to some of my new EM classmates who are straight out of medical school.
If you are prepared to take a training time out for 4-5 years and want some unique life viewpoints, then maybe it's for you.
Don't count on starting EM early, though.
And really look into navy (or whatever branch) EM residencies. Remember that the military population for the most part is healthy, drug free, and has easy access to medical care. Yes, there are the dependents and the retired, but exposure to the general population is at a minimum (on outside rotations), and your patients seen numbers are lower. And also remember--military EM physicians have the skills to pay the bills/man the sand pits and hospitals in combat.