My local library has separate areas for shelving genre books. This is sort of annoying, insofar as it makes me pay visits to three or four discrete areas of the library (mystery, SF/F, general fiction).
The library in the town where I grew up didn't shelve separately, but they did mark all the genre books with colored stickers on the spines. That was great---it allowed you to browse the maximum amount of variety but still quickly zero in on genre books if desired. It also prevented the need to speculate as to where something like a detective story set in a multidimensional alternate Europe would go.