They are derived from the military style boot camp exercises and are much more enjoyable when you don’t have a drill sergeant yelling at you. One of the best things is the team building and camaraderie that comes from completing the event.
I have done some Spartan Races. They are fun but also tough. I would also suggest the Metro Dash races. They are much shorter that Spartan or Tough Mudder races but very intense. They were developed by Navy SEALS.