I've done one Challenge, shadowed another, and am signed up for one in March.
I prefer them over Tough Mudder. It's more physically and mentally demanding, and the team atmosphere is cool. The Cadre members are good dudes.
I like to do some type of event like this a couple times a year just to keep me motivated in the gym. GoRuck uses everything you have so a well rounded strength training and cardio routine is good, and that's my ultimate goal anyway.
It's mostly mental. I signed up for the first one on a whim the day before the event. Training will help, but as long as you're in decent shape you'll make it through the night.
Sign up with some friends and have fun.
The opinions posted above are my own and do not represent any current or former employer.