The camera has been very well reviewed on a number of great sites but reviews do not ever tell the whole story.
I have tried the camera under various conditions ranging from bright sunlight to darkness. In every instance I have got shots worthy of retention even though the subject matter has not been. The standard factory settings work for me but I do change the color space setting from sRGB to Adobe RGB. I think this is better if you are using PhotoShop.
I have the 10D also (what a great camera) and will keep it as a 2nd camera but the 20D will now be the the camera to use. With the 10D I am able to get excellent prints of up to 60 x 40cm on paper and canvas (cloth) which is an excellent medium for printing wildlife photographs to be framed. The 20D should, I hope, boost the sizes possible considerably - I will be trying 80 x 60 shortly.
One major gripe I have is that to sell lenses they (Canon) seem to have lots of kits but no body only stocks in South Africa. This forces you to buy a mediocre lens which you do not want or its a case of wait, wait & wait some more. The BG-E2 battery grip is about the only thing that you need buy extra, again the waiting game.
As regards CF Cards, 512s are ideal as 256s (OK with the 10D) seem to run out of space too quickly. I much prefer to use more 512 cards than big capacity ones in case something goes wrong.
The new toggle switch is great for previews and saves time and fiddling. The camera is both faster and easier to work with than the 10D which is a plus to me. Start up time is minimal - this used to annoy me with the 10D sometimes.
Another point is the good pricing for what you get. This is a VERY GOOD body for the price.
One issue may well be that it will likely show up non L lenses badly. This applies to the 10D also. These cameras do need good glass to perform at optimum so these secondary expenses need considering when buying. Cheap glass will make you blame the camera for poor shots as every imperfection will become apparent. Examples: 28~135 IS lens (dog), the cheap EF-S lenses (some dogs), most non-L telephotos, pirate lens makes and others do not offer the performance required except for happy snaps. I exclude some of the Sigma EXs which are great lenses also.
Be prepared to layout a lot on L lenses - there is no cheap quality glass. Also - I suspect that in many cases the 20D locks up because non-Canon lenses may not be compatable. If I were Canon I am sure I would try to make life difficult for Sigma!!!
I suggest the Canon 100~400L IS USM lens as being a good starting point for birders, you can add more Ls from here on.
Update:
Having now taken some 3600 shots I have felt it necessary to downgrade it from a 10 to an 8. The camera does need the battery grip to feel balanced with long lenses, otherwise its too small for large hands. This obviously has an effect on handheld shots and Canon here have got no stock. Canon have already botched their software update once. The camera has locked up - my 10D never ever gave a hint of trouble. I also find more processing is needed. Perhaps the new software will correct this. My love affair with the 10D is something the 20D is really going to have to work on.
2nd Update
The Ver 1.0.5 software has definitely cured my lock-up problems and a few other minor things - upgraded to 9/10.
3rd Update
My BG-E2 grip has finally arrived after begging for it from Canon (and I mean getting on my knees). It is almost unbelieveable what a difference it makes. The 20D suddenly becomes a whole camera, balance with long lenses beomes natural and hand holding at 400mm becomes a pleasure. Now I can give the 20D its maximum points.