The biggest thing you need to understand is that when you join the military you are committing to train with the military. You will be applying to the military residency system at the end of medical school, rather than going through the civilian match. If you want to do the civilian match you can ask them to let you, but they generally say no to such requrests.
The military match does not have the same odds as the civilian match. For a few specialties the odds are actually slightly better than in the civilian world, for most they are about the same or slightly worse, but for a few (emergency medicine in particular) the odds of getting what you want are WAY worse than in the civilian world.
The other thing about the military match is that not all residencies are categorical. Some military docs will only match into an intern year. At the end of that intern year they apply again to train for the rest of their residency, and many of them will not be accepted. Those who are not accepted will begin working in the military with only 1 year of post-medical school training, and will practce a very limited form of primary care until they can reapply 2 years later. At the moment the Army has the most categorical residencies, followed by the Air Force, and than the Navy is a distant third. The Navy claims that by the time you graduate they will be in second place with the AF in a distant third, but no guarentees.
Finally please understand the payscales to make sure that this is the best way for you to join from a financial perspective. If you're going to a cheap state school you might be better off joining later in your career.
