I've done the Ohio Backyard (one of the regional races that can now qualify you for Big's) twice and the format is really interesting versus regular races. Speed does nothing for you in a Backyard race, unless you need a longer rest interval to take care of business (change clothes, porta-potty, sleep, etc.). A backyard race is something anyone can do - people in the race had different goals and everyone ran together at the beginning of every hour. Some people were looking to cover a marathon distance, some 50k, some going after their first 50 mile run, etc. Compared to a traditional race, it's a low-pressure way to cover some distance and have some fun, and when you've had your fill of running you pack up and go home.
I wasn't interested in a blood & guts duel to the death, I just wanted to experience the format and see if I liked it. 12 hours on the trail course, 12 hours overnight on the road. It was great - the first time I did it, I wanted to get some trail miles in as part of my training for other races. Once it got to the road section I thought "I'll just keep rolling because this feels easy and as long as I stay warm enough, I'll keep going," Once we switched back to the trail, I realized a lot of people would drop out at 100 miles so I went for two more hours - I could have kept going, but I got out of the event what I wanted and I had to get home and do my chores! The 2nd time I did it was just for fun - I got through the trail miles, did one road loop and went home (thank God because it started raining like hell!).
Not every race or event has to be a goal race - the backyard format is a cool way to do an ultra for ANYONE, and it's a great way to get some training miles in for another goal race (or sleep deprivation training). I'm no speedster, but it took a few hours to relax and just float through the early part of every loop and chat with people who were just looking to go 50k or 50 miles. I'm sure once the field gets whittled down, there's a little more competitive fire, but early on you find yourself rooting for people to hit their goals - in a regular race, those folks would have been way behind most of the field but with the whole field starting together every hour, you got to know the other people and you started pulling for them.