Yeah, let me rephrase that to say that if you can tolerate the military, you're likely to stay for at leats 20 years. The people who don't and leave are further evidence of people who didn't like the lifestyle for whatever reason.
Either way, it's not an easy decision.
I don't think that it would be accurate to say that if you can tolerate the military, you're likely to stay for at least 20 years. That would be like saying that you can tolerate a certain hospital you work in, so you are going to stick around for 20 years.
Even before the current "wars" physicians would get out of the military in droves. The reasons are many, financial, family, more latitude in location as a civilian, etc etc. Some people who joined years ago, joined under the false assumption that they would not deploy because the US hadn't really been involved in any drawn out conflicts for years...if you join today, you will likely deploy at least once during your payback time.
Jolie south, megrimlock and spicedmanna give some solid and accurate advice in their previous posts in this thread.
If you never would consider serving in the military, don't even consider taking one of their scholarships.
It isn't for everyone. As a matter of fact, not every one can cut it in the military anyway, even if they can "qualify" on paper for the scholarship. That said, as a medical officer, your quality of life will be extremely for the most part when compared with anyone else in the service.
There are plenty of trade offs associated with the scholarship and these are things that each person needs to weigh based on their own situation.
I served for 4 years as an officer in the army, separated to attend medical school, and was very cautious in signing up for the scholarship (even though I knew I could already hack it in the military and knew full well what I was getting myself into).
I weighed all my options, looked at pros and cons of taking loans vs military scholarship (and of course compared the various branches). In the end, I decided the scholarship was the best route for me.
I enjoyed serving, I enjoyed being apart of a group of people committed to each other, and I enjoyed traveling around the world. It wasn't all great, but no job is, but it was interesting, exciting and something I am proud of. It sounds corny, because you never really feel special about your service when you are surounded by other military folks, but when you step outside that bubble, and people find out that you served in the military deployed to Iraq/Afghanistan/etc, they treat you like a hero.
But, like I said, it isn't for everyone and the military med threads on SDN will give you a good feel for what it is like to join (not knowing what you were getting into) and being really bitter about your time in the service, but there is truth to most of the posts there and tons of good information as well.