I did it for the first time last year and I'm signed up to do it again this year. I did it in part because it scared the hell out of me and I like to do things that scare me, to break me out of my comfort zone. Basically, you have a team of 12 people and you will run 3 times (legs.) It starts on a Friday morning and goes until Saturday afternoon, so one leg will likely be at night. Legs vary in distance from about 2 miles up to maybe 9-10, and the terrain can really vary. Your captains will generally assign the slots based on ability. My combined mileage last year was about 15, which was about the middle of the range for my van. I will likely take on more miles this year. (my longest leg was 7.8 miles - very hilly - and shortest leg was 2.2 miles.)

Some things I learned:
(1) Even if you don't know anyone on your team except for your van captain, you will know everyone in your van REALLY WELL at the end. I am now facebook friends with everyone in my van, and we talk regularly. You don't mingle much with the other van/teams, but people are pretty supportive of other teams. It is a very cool/fun group, in general.
(2) Don't get too attached to the legs you've been assigned. My group changed mine THE MORNING OF the race because someone was dealing with an injury.
(3) Running at night is less scary than I thought it would be. It was actually awesome.
(4) Don't overdress because you think you'll be colder running at night. I did this, and had to try to take off my long-sleeved shirt, which got tangled up in my reflective vest. I almost choked myself to death on the front lawn of someone's mcMansion at 2:00 a.m. trying to untangle myself. My team thought that was pretty hilarious, but then again, we were pretty sleep-deprived.
(5) You won't sleep much. I got 1.5 hours, plus maybe an hour nap in the van.
(6) You might not shower much. We rented a hotel room so we all got a shower between the 1st and 2nd legs, but not before the 3rd.
(7) Vans really smell when there are 6 people in them who are bathed in old, dried sweat.
(8) The third run is the hardest (at least it was for me), regardless of the distance, because you're just really tired and there are lactic acid buildup issues.
(9) You get a free beer at the end, but you're less excited about it than you expect to be.
(10) You will likely fall asleep by 8:00 that Saturday night.

You'll want to do it again. (Also, buy your own headlight and practice with it beforehand, and it's best to wear it on a visor or cap. I borrowed one from a teammate and it was too big so I kept messing with it, while trying to run on a rural road in the dark. So learned my lesson.)

I have friends who have done the Ragnar and loved it. It sounds like loads of fun and not too much pressure, aside from possibly your team. It all comes down to your teammates.

DH and I kept thinking about it but were a little intimidated by it, especially now with 2 kids under 2. But we have a good friend who did it and he said it was a blast! Just get a good team together who know your pace and aren't worried about the team time. I think the team just splits up the legs so see if you can get some more experienced runners together to run the longer legs, and see if they'll let you do the shorter ones. Don't worry about competing against the fast runners- there will be people of all paces and experience levels there. Good Luck!

anyone did a regnarelay before? sounds interestingly different and takes novice runners. how was it? do all participant have to run all days? hate to compete with the fast runners and slow everyone down.
http://www.ragnarrelay.com/