Two products stand out from the crowd -- Epson's Perfection 1640SU for its combination of good speed and excellent image quality (thanks in large part to its 1,600dpi optical resolution), and HP's ScanJet 5370C, which is an excellent all-rounder at a good price.
Of the remainder, Microtek's ScanMaker X12USL Pro impresses with its USB and twin-SCSI interfaces, but its scanning software could be easier to use. Umax's Astra 6450 is the only product with an IEEE 1394 (FireWire) interface, but this fast connection isn't really exploited. Visioneer's OneTouch 8600 is admirably easy to use, but (like the Umax Astra) has only a 600dpi resolution and features neither a transparency adapter nor an auto-document feeder option. Finally, Acer's ScanPrisa is extremely slow compared to the other products reviewed here.
Our overall recommendation is the Hewlett-Packard ScanJet 5370C, but the Epson Perfection 1640SU is well worth considering if image quality is paramount.