I like having two camera bodies, and the 1.6 crop and full frame are complementry. I basicly have a light weight setup in the 20D with the lighter S series lenses, along with a 70-300 Is. It is lighter then my 5D, which I pack mostly with the heavier L telephoto's. Of course, if I am not worried about the weight, I can use the 20D with any of the lenses or use both bodies as i did from my car this week.
The IQ on the 5d is outstanding, it is better then the other. But the 20 is not shabby either. I especially like the 5d with landscape, as you would expect.
Another reason for my using two bodies, is that my underwater setup would be expensive to do over - my housing is for the 20D, and I do not want to re do it.
The controls are so similar between the two bodies, that it is intuitive also.