I use graphtech nuts in all my guitars. It is impregnated with Teflon which means that you don't need to lubricate it with graphite or other kind of lube.
For guitars equipped with tremolo, bigsby or floyd rose type bridges, it helps a lot to keep the guitar in tune (make sure you have no other parts restraining the strings like bad string trees, etc..). In acoustic, classical and fixed bridge electric guitars, it makes tuning easier, prevents string breaking and helps to keep tuning when doing bends.
It looks more rigid than any other material that I've tested (bone, plastic etc), but it's not hard to sand to achieve the desired height.