First, I'd caution that if you see HPSP as just a way to pay for school, you'll be disappointed and maybe downright unhappy for 3-4 years. If you like the idea of serving your country and the idea of some unique experiences prior to settling into a long term position, it may be the thing for you.
My experience:
Signed up for HPSP in my first year. They pay a monthly living allowance, ALL school expenses, and all books/equipment (which are yours to keep after leaving the military). Did my officer basic training between 3rd & 4th year (exercise in AM, classroom all day, and on the golf course by 4:30).
Did 2 of my 4th year rotations on military bases, so I was paid active-duty pay for 1/2 of my 4th year while everyone else was scrambling for loans to cover the last 6 months of school.
I listed 3 places I'd like to be stationed, and was assigned to one of them. Took 1 month off & then could start practicing mid-June before the rest of my classmates were even licensed.
Military optometry allows you to practice full-scope for procedures/prescribing and you'll either be assigned to a large hospital where you work alongside ophthalmology, or to a smaller clinic, where you may be in charge from day one. I got to experience both and saw more pathology than many of my friends doing residencies.
You will be expected to participate in some non-clinical military activities, but as an O-3 officer, wont be assigned many duties. During my 3 years of active duty, I saw patients from 8-4, took Wed PM off and was on-call for the ER every 3rd week. About once a month would take a day for weapon training, attend military lectures, etc. I spent 3 weeks in Haiti doing humanitarian aid and was also EMT certified. All national holidays were 4 day weekends and all CE trips were paid for. I had accumulated 80 days of paid vacation when I left, which allowed me to get full pay for the first 3 months in my private practice.
My undergrad was paid with academic & a football scholarship and my only student loans were $25K for the 1st year of opt school. When I left the military after 3 years, Id saved enough to make a down payment on an existing private practice and a home. Overall, a great experience!