Photography nerds have -- like most other nerds -- a tendency to want to try the extreme. If you're not that kind of nerd, move along.
Macro
So after a glass of whisky or so, we decided to make an insane test. Odd Einar has a the EF 100-400 mm f/4.5-5.6L IS lens, as well as the matching 500D close-up lens. I have the EF 1.4x Extender II, and a 20D which yields a nice cropped frame view. Perfect for macro shots!
So we took a shot at approximately 1m distance of the coat-of-arms on the Laphroaig bottle in this image:
This is what it looks like at reduced view:
Fun, eh?
Fisheye
The Canon EF 15mm f/2.8 Fisheye is close to useless on a 20D, but with Odd Einar's 5D, it was a lot more fun. What surprised me is how sharp it is. Well, at f/5.6, anyway. And it makes for lovely(?) caricatures, such as of yours truly:
Except for making lenses work as they were designed again, the 5D truly is a lovely camera. It's everything the 20D is, and then some. I hope to use it some more, but not enough to convince me that I need to buy one. 
D2x
No, I haven't changed camera brands and bought a Nikon D2x. That's Paul's camera, which he let me test for a little bit. It's a nice camera, but the controls are so completely different that it's hard to get a decent impression.
The viewfinder is brighter and definitely a step up from the Canon EOS 20D that I own. The camera fits my hand nicely -- again better than the 20D -- although not quite as well as the EOS 5D.
Image quality seems superb -- at least under the conditions we had in Singapore Botanic Gardens -- but I've yet to see anything but what I get on the rear LCD.