I've met many people who have served in various branches. I'm my experience, obviously anecdotal: people in the Air Force and Navy are mostly regular people. The army can go either way; they're largely undereducated but otherwise regular people. Marines are not normal people; they are brainwashed jarheads who believe they're god's gift to the world (and to women).
In my experience, marines are either some of the best, most down-to-earth, coolest people on the planet, or the most absolutely insufferable jarhead assholes you could imagine, and there is absolutely no in-between.
By and large, the cool ones tend to have not enjoyed their time in the service so much, they don't necessarily regret it, there were probably some bright sides that they enjoyed and can look fondly back on, but once they're out they're glad to have it behind them and move on with their life. The assholes tend to look back very fondly at all of it and think of it as the greatest thing they've ever done in their life (and sadly for most of them, they're probably right.)
It also tends to divide along if you had some actual skilled job, or were just some menial labor cannon fodder. If they worked on aircraft or vehicles, or were an armorer, or a tech person, etc. they're probably one of the cool ones.
The assholes tend to either greatly outnumber the cool ones, or the cool ones just tend to not be so damn loud about it. Generally speaking I end up assuming anyone I know was a marine is a trash human being until they prove otherwise (3 people who I consider to be some of my closest friends are marines, but marines are also way over-represented in my list of worst people I have ever met)
The absolute coolest people in the military have to be the coast guard though. I have never met a coastie who wasn't an all-around great person.
All of the military is brainwashed in some way or another, that's literally what boot camp is designed to do. It's just a lot more basic in the non-marine ones.