I also had the urge to upgrade the 20D to the 5D. After much thought, I didn't take Canon's bait.
Upgrading to the 5D was hard call for me as well, but no looking back now!
The AF on the 5D is seems perfect, AF had always been my biggest issue before. The images look great too. Not sure if its the 12MP or a different AA filter, but my out of camera shots now need no or very very little sharpening, that was not the case before.
I purchased with the option to return it, but quickly decided its a keeper and sold off my EF-S lenses and camera to pay for the upgrade.
The only downside is if your doing heavy cropping for bird shots is the lower pixel density but that would not show unless you printed over 9" wide anyhow.
. Personally, I think it's really silly to allow the 1.6 crop factor to be a gating factor in upgrading to the 5D from the 20D. The money issue is valid though. The 20D is much much cheaper and the money saved could be used to buy some really wide lenses to make up for what's missing on that end and more. I personally think that you could crop to the same FOV without much if any loss in quality. Anything is possible in Photoshop 


