I only have a small grinder...its wood with metal rods, and a really really awesome roundish shape that's fun to play with, but I have to pretty much pick the weed out and there's always plant matter stuck on it no matter what, probably since its wood, its a nice one though.
It entirely depends on the weed if I'm grinding it. I always smoke from a glass bowl or a glass on glass bong with the occasional blunt, and for blunts I think it makes life much easier. Since I usually smoke alone, I usually just use my hands to break up the weed for the bowl/bong since it's fewer steps and just easier. However, on occasion, I get this ridiculously dense nug, it's decent weed but its hard to deal with, so a grinder is pretty much necessary for that --- even if it's decent, sometimes I turn weed that dense down for something a little easier to deal with.