The Brother DS-700D ($200 street) is the least expensive portable scanner we've seen that can scan both sides of a page at once. This ability to duplex may be enough by itself to make the scanner worth considering, particularly if you're on a tight budget.
Gets power over USB cable; Duplex scanning (both sides at once); Comes with protective sleeves for scanning easily damaged originals
Manual feed only; Large and heavy for a portable scanner; Severe limits on scanning multiple document pages into one file