It's amazing to look back just a few short years and realize how much has (and hasn't) changed.
I bought a 5D in 2007, and relegated my 20D to back-up status. When traveling, I frequently carried both bodies: the 5D with a wide / normal range lens, and the 20D with a telezoom, since the 20D offered higher resolution than an APS-C-sized crop of the 5D RAW image.
I skipped the 5D2 and bought a 5D3 about a year ago. The features I like most are the AF system, locking mode dial and repositioned and resized DoF preview button. Most of the time, I shoot in MRAW, since 10MP is enough for the vast majority of my applications.
An APS-C-equivalent crop of the 5D3's sensor is higher resolution than the 20D, so nowadays when I travel, I use the 5D with a wide / normal lens, the 5D3 with the tele, and the 20D sits on the shelf. I keep hoping that a niece or nephew will want to use the 20D as a starter body for dipping her / his toes into SLR photography. When I want longer reach than my lens will afford (and when I remember to do it), I switch the 5D3 resolution to full RAW, and crop those 22 MP in post.
I'm looking forward to the inevitable price drop of the 5D3 when the 5D4 is released. Short of a miracle in new technology, I'll be happy to pick up a second 5D3 for about $2K, retire the 5D and call it a day.