I write this having originally joined the Navy via NUPOC. I did one junior officer tour on the USS Charlotte and then a shore tour teaching NROTC at SUNY Maritime where I took organic and biology at Columbia at night. During this time I also spent five months at Kandahar Air Field with a small 82nd Airborne logistics task force. As of writing this I'm a MSIII at USUHS. I'll accept that I've gotten almost everything I've ever asked the Navy for. Thus, when I haven't gotten what I asked for, it's been relatively easy to accept that my desires did not fit with the needs of the Navy even when those needs were noticeably in part due to individuals poorly managing human resources. I think that it helps that I've generally tried to have a positive attitude about accepting that the needs of the Navy come first although the Navy tries to keep its people happy by giving them what they want. One also must accept that as a massive institution the military has plenty of faults. However, even with all its faults, I think it is one of the best, if not the best, institutions in the world due to the nature of its social contract.
The rest of this is just my opinion based on my experience and the asking of lots of people lots of questions about how things actually work. You are free to disagree and comment if you think differently. Further, you will find lots of people on the internet who will disparage the military and the military medical system. Many of their critiques are valid, but nonetheless, I think the rest of the options are worse. In military healthcare system no doctor will even make a clinical decision based on its financial benefits to him or her despite its negative health, welfare, and financial benefits to the patient. I've written this for one friend who specifically asked about HPSP, but it could be useful to lots of other people, so I've posted it here.
The active duty commitment is only one year of each year you get the DOD to pay for medical school, so if you have them pay for all four, you owe four years of active duty not counting residency. Any contract with military is for an eight year stint, with the number of active years specified. So if you owe four, you get out with owing four years of inactive reserves. This means that you are on a list of people who don't drill and the secretary of defense can force you to return to active duty, usually for one deployment, without actually drafting you. Traditionally, this has been nothing, but in the last few years some Army and Marine trigger pullers have had it done to them. I don't believe any doctors have had it done to them. If you do a longer residency and/or fellowship than four years, you also add time on for that, usually on a one for one basis. So if you do an ENT residency, five years, you then owe five years, plus the four for medical school, but they can be served concurrently, so you've really only added one year to your commitment. Also, the time spent in residency or fellowship, if on active duty, counts towards your twenty for retirement. If you do a civilian residency/fellowship, then you stay inactive duty, pushing your four year commitment for medical school to the right. So if you owe four for medical school, and do a five year residency, you only must serve for four years afterwards. The downside is that you received civilian residency pay of ~$40K while a military resident is paid based on his or her rank. I expect to make about ~$100K as a resident no matter what my speciality. You can also do a residency, then a tour as that speciality, and then go do fellowship or another residency without facing any pay cut. Of course, you do extend your time where Uncle Sam gets to tell you where to live.
I would recommend the Navy over the Air Force or Army. I think I've got good logically reasons, which I'll explain, but I'm certainly a bit biased by a relatively good ten years in the Navy. If you really want to run around in the dirt, the Army is a good choice. For instance I have a classmate at USUHS, who is not the super strongest academically. (I'm not saying he's dumb; he's a perfectly good medical student, but no one considers him neurosurgery material.) Although I haven't seen it, I wouldn't be shocked to notice him snacking on old shoe leather in the library because he likes the taste. He plans to become a special forces doctor, likely via family medicine, and he'll be a better special forces doctor than someone who is neurosurgery material any day. If you really want that, the Army is the best option. The Navy and the Air Force can both give you that, but there are a whole lot fewer opportunities for it. Another significant traditional difference in the Army is that it's been a stronger promotor of research by physicians. This has been possible because of the shear greater size of the Army and may or may not change with Walter Reed closing and moving to Bethesda to combine with the Navy. The move could make the Navy, and even Air Force, more supportive of research or could make the Army less supportive of research. I've heard different opinions. A third advantage of the Army is that its size and need for surgeons probably offers the best chance for someone academically weak to obtain a surgery residency. If you don't match, you'll be assigned a residence based upon the needs of the service. My understanding is that the biggest amount of unfilled residencies have recently be in surgery. The big disadvantage of the Army is that, if a war is going on, you will likely have the greatest chance of spending a year living in a tent. Conversely, if there are no wars, the worst you'll experience will be some weeks of field exercises and perhaps a one year unaccompanied tour to Korea. I know one CNRA who never deployed from her commissioning as a nurse in about 1994 to about 2004. She then did two six month deployments to Iraq as a CNRA, where she saw some pretty serious stuff and faced rocket and mortar attacks.
The Air Force is traditionally considered the best option if your number one concern is quality of life. You will have to deal with people pointing out to you that you're really in just a world wide country club than actually the military. Its also known for being strong in family medicine. Not that family medicine is more competitive in other services, but that in the Air Force other specialities are more competitive because the Air Force wants lots of family medicine docs. With family medicine you can be stationed anywhere, and I have heard of people getting places like England whereas a its almost impossible to get stationed there in the Army or Navy. The Army does still have a bit in Germany, but not like during the Cold War. The Navy has plenty of billets in the Pacific such as Hawaii, Guam, and Japan.
I think the Navy offers the best option of playing in the dirt if you want it, with the Marines, but then the option of not doing that if you don't want to anymore. Because we're on the coast we generally have the best places to live. Our big three hospitals are in San Diego, Bethesda, and Portsmouth. San Diego and Bethesda both have great quality of life, and even Portsmouth, about which I had low expectations when stationed at the shipyard there, was way better than I had expected. There are a few more out of the way possible duty stations, but if you'd like to avoid them, it's probably possible. Although you could be a flight surgeon after internship, but before the rest of residency in the Army of Air Force, they have few enough, that you likely won't. You will likely go straight through residency like in the civilian world. The Navy still has many many general medical officer (GMO) billets. This is like a general practitioner in years gone by. For years there has been talk of ending the billets, but so far note much has changed. In a straight up GMO billet you do physicals and sick call. Quality of life is good, but interest of medical practice is low. Some people can go straight through from internship to residency, but this is only determined during PGY-1, when you apply to PGY-2 onward and GMOs at the same time. Besides basic GMO at some place like boot camp, you can also do Marine battalion surgeon, undersea medical officer with the submarine force or divers, flight surgery, and shipboard medical officer. In all these billets you'll do physicals and sick call and serve as the medical officer, the person who advises the commanding officer of medical issues amongst his troops. Any thing the commander thinks is medical, such as pest control, is likely to be assigned to your responsibility. Think of these billets as you being a military officer first, who must be a qualified doctor to adequately perform your duties. This is in contrast to a pediatric general surgeon in Bethesda, who is primarily surgeon, but wears a uniform when not in scrubs. Many people want to do GMO because it gives them a chance to see the Navy and potentially deploy. If you deploy with the Marines currently you'll fly on an airplane to Afghanistan, but traditionally you would float on a ships for six months pulling in to liberty ports waiting for the president to need some big guys with guns in a third world country moving towards chaos. The battalion surgeon is responsible to make sure the minimal number of Marines come back to the ship with STDs and other more innocent infectious diseases such as malaria. For some competitive subspecialties, such as ophthalmology and dermatology, it is de facto that you must do a GMO tour first because their competitiveness necessities the extra points in being accepted.
I won't detail the points system, partially because I don't really understand it, but it's the system by which the military formally weighs candidates for residency programs. Each applicant is give certain points for grades, boards, research, GMO, and the such. It's pretty opaque to us, but it is what it is. Unlike the civilian match the military match is not done by a computer. It is done by all the program directors sitting in rooms and deciding where each person will go. It is done in December, so that those people who don't match in military programs can then enter the civilian match.
All branches require you to go to some form of initial accession training, akin to boot camp, but not. As a doctor you will be commissioned either before you show up or when you show up. You've earned your commission by being physical qualified and being in medical school. You can go anytime during medical school that you can fit it in your schedule or you can go when you finish medical school prior to starting you career. In the Army you'll have to do things like field exercises. In the Navy you do some PT and learn what the ranks are in an air conditioned classroom in Newport, Rhode Island. You may initially get yelled at, but after a bit you'll relax because the people running it are not really interested in expending enough energy to yell all the time. It is not hard. If you have trouble making it through, it is a safe assumption that you are trying to not make it through. The Navy will then either cause you pain to stop being stupid or make things easier so you can't not make it. Other than the scare tactics at the beginning, you will not be treated like feces.
In general as a doctor you chances of being hurt or killed in the military are pretty pretty low. I believe that four doctors have been killed in Iraq and Afghanistan. I don't know how many man-months doctors have spent in country, but four doctors in that many man-months could have died from anything. I doubt the risk of being a western military doctor in Afghanistan is higher than that I accept by riding the notoriously unsafe Washington Metro to work most days.