I have had excellent experience with the Fujitsu ScanSnap fi-5110EOX.
Great for scanning big stacks of paper.
It's purely sheet-fed, with a 50-shet ADF, not a flatbed.
Street price is about $400.
It scans about 12 double-sided sheets/minute at 300 DPI,
and 6 sheets/minute at 600 DPI (these are my actual measured
speeds, not specs).
The thing I like most is that it's unflappable: never misfeeds,
never makes me reboot to get its attention, doesn't wander off
into a black hole, responds gracefully to errors, etc. This is
a marked contrast to the other somewhat faster but far more
persnickety scanners (Xerox Documate 252, Canon DR-2080C) that
I am also using in a large document-scanning project.
It comes bundled with Acrobat 6.0, as well as its own bundled
scanner interface software that is stable and simple to use
(again, in contrast to many other units).