I have some strong opinions & some ambivalent ones about this...
I think most young folks would gain an awful lot from military service and think that our nation should implement a mandatory 2 year obligation of service. Not limited to military service, options for community service, etc. seem like a good idea for those not inclined for military service.
I've been a civilian for 25 years, so some of my opinions are probably outdated or irrelevant, but I think that military service is a defining aspect of a young person's life. This is not always a good/great thing. If the individual is not a good fit for the branch of service or job assignment it can be a limiting situation that holds the person back from success. On the other hand, if the individual is at least cognizant of the situation and performs adequately enough to get an honorable discharge it will probably help them in future endeavors.
That being said, my experience was that the USMC was probably the best decision/opportunity that I dumbed my way into. I dropped out of college and decided that I needed to be self supporting so the military was an instant version of 3 hots & a cot without MOM to harass me daily. I talked to all of the 4 main branches (sorry CG...) and got the best opportunity from the Corps.
I was not prepared for what boot camp entailed, but luckily I was given advice from a friends father. He told me the day before I left for boot camp "never be first, never be last, and never volunteer." There was no further explanation, but it became clear very quickly...
Additionally, I realized within the first week of boot camp that it was essentially a mental game. Their goal was to decide which recruits were able to withstand extreme pressures and which were not. If you were unable to take the mental pressure of boot camp, you would be a liability in combat. I was pretty fit physically and so I found the physical aspects were relatively inconsequential compared to the mental aspects. But once I figured out that it was a mental "game" it became very easy to deal with.
As stated by others in this thread, there is a "mentality" involved in joining any branch of the military and nearly everybody that joins the Corps is a Type A personality (whether they know it or not)...so Marines are always competing. This is one of the great things about the Corps and also one of the reasons that some don't succeed.
All that being said, in my experiences the USAF had the best facilities and living conditions, followed by the USN, and then the USA. The USMC had the worst facilities and living conditions...
So, although I harass my friends that are not Marines, I respect them and think highly of all people that have served in any branch of the military.
I was probably the last person my friends in high school expected would join the military, but I would not trade my experience serving in the military for anything. It made me the individual I am as an adult and has made my life better for the past 30 years.
Edit to add:
My signature line probably describes it best. That was the case prior to my becoming a Marine, but it accurately describes all the Marines that I know...