i have enough problems with new scanners, i'd be afraid to get a used one! but for the better price, it will probably last just as long. typical, a scanner from 1988 that scans 72dpi in 15 minutes still works, but anything new only lasts a couple years.
my last scanner got worse and worse. dust on the mirror, lines, yellow scans. after taking it apart like 5 times over a couple years, i finally couldn't find any way to fix it anymore. that was a canoscan. now i have an epson 4490. i guess it's 'acceptable'. it's not a bad scanner, but it does a lot of things that bug me. and the canoscan did a much better job of scanning poorly exposed images that i could correct in photoshop. the epson doesn't pick up a lot of things.
mid-level scanners are going to become extinct, i think. it's going to end up being those crappy copier/scanner/printer combo things and high-end scanners for businesses. they are already relegated to a back corner at fry's and best buy doesn't have any on the floor.