I did this ONE time and under strict guidelines that I was a *backup* photographer. They were to hire a professional, and I would also take photos and they could buy what they wanted.
To be perfectly blunt, it was what some might call a "redneck wedding" on top of it, so, when they hired Uncle Billy Joe to be the main one, he brought a Kodak point and shoot with him to do it with. 90% of his shots were too dark to even make out who was getting married, and the ones that were somewhat fixable had so much noise in them even software couldn't help them much.
I used my 10D and most of my shots turned out pretty darn good. Since he was the "hired" photographer, and I was the backup, he was the one who turned into "the one who ruined the wedding photos" and I was the one who saved the day. :G