I realize that "affordable" is subjective and also that part of what makes something a "good deal" is fitness for your workflow, but I love my Canon P150 (apparently reincarnated as the P215) because it's duplex, portable (usb powered) and the software gets out of my way.
I don't have a lot of experience with full-size automatic document feeders to compare, but I wanted to at least make you aware of one happy customer.