I just purchased a Canon 5D Mark I. I upgraded and downgraded at the same time. I've spoken to several photographers that have said that even though they all work with a Canon 5D Mark II, they would still endorse the Mark I and still is better than any crop frame out there. So, I tested it out. Gotta say, I'm not mad at the outcome at all =). BTW, my daughter is insane!
Photos taken by yours truly =)