Not personally, but a friend of mine in Texas did one with a friend this year. They dressed up in all pink matching fitness outfits and wore their hair in pigtails. They had a blast, though I have a feeling their outfits were a total loss -- they were totally covered in mud at the end! They were both slow runners who didn't do much more than a 5K distance, so it is certainly fine for all fitness levels, I'd think.
I hate mud -- walking in it, hiking in it, or mtbing in it, so I don't think this is something I'd ever do. But they say it's lots of fun if you don't mind getting dirty!
Emily
2011 Jamis Dakar XC "Toto" - Selle Italia Ldy Gel Flow
2007 Trek Pilot 5.0 WSD "Gloria" - Selle Italia Diva Gel Flow
2004 Bike Friday Petite Pocket Crusoe - Selle Italia Diva Gel Flow