These legacy to USB devices may or may not work. It's not just that some of them are less than perfect. Some SCSI scanners, specifically Nikon models, state that they will not work unless directly connected to a SCSI card. This has led to fierce competition for USB scanners when they come up on e-bay.
It's a pain (though I benefitted from Apple owners chucking out scanners of all types when Mac models dropped SCSI). If you have a desktop computer I would suggest fitting any old Adaptec PCI card you can find cheaply.
Alternatively look for a Plustek scanner -- these seem to be the only reasonably priced USB scanners which work with both Apple and PC. http://www.jessops.com/online.store/products/76664/show.html