I'm considering a 5D to augment the excellent 40D, which in itself was an upgrade over a Rebel XT. I've come to appreciate the button layout of the 40D. Everything seems like it's in it's right place. The ISO button next to the shutter release, ISO in the viewfinder, and the AF-ON are all perfectly placed.
For users using both the 5D and 40D, are the 5D's ergonomics and button layout difficult to adjust to after using the 40D?
No, they're actually pretty similar. The menu layout is the same style as the 20D, with a long list that you can scroll through. It doesn't have the 40D structure which has 5 or 6 different pages. There are advantages to both. Otherise most of the knobs and buttons are pretty much in the same place.
The 5D displays ISO in the viewfinder, but only when the ISO button is pushed, whereas it's there all the time in the 40D.
I don't find it difficult to switch between the two cameras