Current Air Force HPSP physician so some slight differences than army but also similarities. I would only recommend HPSP if you absolutely truly want to serve in the military. Any other justification you use has the potential to make your time in the military miserable. It's very different than the civ side on multiple fronts.
Once you're in med school if you go the HPSP route, you'll do officer training (I think Army is BOLC). Then once yearly you'll be required to do some sort of active duty tour. For me I did commissioned officer training the month before med school, 2nd year I did a 2 week aerospace medicine course in Dayton, OH. Then I did 2 rotations as a 4th year at military treatment facilities as away rotations in the specialty I wanted to go into. Other than that you're pretty much straight civilian for med school.
You're limited in the amount of overall training spots just due to the military's residency training having overall less sites and spots for pretty much every specialty. You'll be competing with people who are getting out of school just like yourself but you'll also be competing with people who do a general medical officer/flight surgeon tour before residency and they will be given priority over you. This is an option especially if you're looking at a competitive specialty as you get more points for doing GMO/FS, but you also are delaying your training for a few years. Once you're in residency (depending on where you do your residency) you could be limited on the training if say you're only seeing active duty patients or dependents in training (could be specialty dependent as my residency was split military/civilian and we got great exposure on the civ side).
Once you're in practice, the military will decide on where you go. You get to fill out a dream sheet of places you want to go but ultimately the mil decides. You could get your #1 choice or you could get sent to middle of nowhere. Once again depending on specialty, you could struggle with skill atrophy depending on where you're sent as you most likely will be seeing strictly active duty, which are already a preselected healthy population of people that must maintain certain health and physical standards, so the pathology you're exposed to could be much less than civ side.
No matter what specialty, you will make less than your civ counterparts. In primary care the gap is less. In the surgical world and getting subspecialized, this could be to the tune of hundreds of thousands of dollars less per year.
You likely will be doing a lot of occupational medicine as your goal is to make sure the patient is healthy enough to do their job within the military. Sometimes this means you have to make the decision that someone is not healthy enough to do their job which could be temporary or permanently disqualify them from active service. They may or may not be happy with you about this. Some patients don't want to get better and they are very aware of financial compensation for being medically retired out of the service so you will see many people who want to remain in the sick role for secondary gain.
There is an ample amount of admin and excessive paperwork. Often times you will be put in a role that takes away from direct patient care such as flight commander, med director, can even work your way up to SGH or Sq commander. It's not uncommon that as you climb in rank, you're taken away from patient care duties to be in charge of admin duties. If that's your thing then more power to you the military is great for that. If you strictly want to be in patient care, that can be challenging at times within the military structure.
You absolutely can and likely will be deployed at some point in the Army. You have to move every few years. Army may be little different on time frame that you're able to stay but Air Force is typically 3-4 years. If you're married with kids, this can be challenging on the fam having to relocate every few years.
On the plus side, you do get to own the sense of pride from wearing the uniform. Overall not many people can say that they've served in the military. Typically where you're stationed, the community is very respectful of you, the uniform, your service and frequently people will come up to you and say thank you (at least that's been my experience everywhere I've been). Also many businesses/restaurants etc will give some sort of a military discount.
There's always a bad apple or 2 in any setting but by and large, I've worked with some great people and am very fortunate to have met and worked with them. Some love mil service, some don't, but it has made my time in clinic much more bearable. You feel bonded to people in which you're all embracing the suck together lol.
Ultimately I am looking forward to getting out. I have less than a year left. I certainly would not have had the experiences I've had or lived in the places I've lived if I didn't join. Would I do it again if I had the knowledge then of what I know now? Not quite certain I could say that.