I was on a 3-year HPSP scholarship for optometry school. I received a stipend monthly (about $900, it's up to about $1000 or so now), and for 6 weeks each year I got paid as an active duty officer on a 45 day rotation (they like you to actually do the rotation at a hospital somewhere, but if you're school schedule doesn't permit this, then you just get paid to stay at school). I also got reimbursed for books and supplies. It was a nice deal for me for optometry school, because my debt is minimal, and this has allowed me to pursue medicine. HPSP for medicine is a little different, because there is a residency involved. If you do HPSP for medicine, you will most likely end up doing a residency in a military hospital.
Of course there are some ups and down in the military service, whether it's USUHS or HPSP. Yes, you could end up being deployed. However, realize that you will be a doctor. Nobody is going to hand you a weapon and tell you to take this platoon forward and charge that hill. Is it dangerous? A little. You may be forward deployed, but it's all for field medicine. Trust me, nobody is going to forget you are a doctor and try to send you to the infantry. There is also another big plus to military residency: the money. It's a more comfortable salary while in residency. Sure, you won't make tons of cash after residency, but it's a comfortable living. If you want champagne and caviar, then maybe military service is not for you. But if you want a decent salary without dealing with HMOs or billing issues, it's not a bad way to go. Another bonus: say you really want a residency that is competive, and you don't get it. After your internship year you could go on for a one or two year assignment as a GMO (general medical officer). This could be anywhere from Bosnia to Korea to the Pentagon. After you do a GMO you could reapply to the residency and you could be more competetive.
That being said, I applied to USUHS. It's almost a no-brainer for me. I already have some military service, and I live in the area already. In fact, I work at Walter Reed, one of the hospitals affiliated with USUHS. If anyone wants to ask anymore questions about the military, the DC area, or USUHS, I will be happy to share what I know.