I like short form-fitting ones like the REI Mistral Gaiter with low-cut trail runners, to prevent grit and pebbles and small furry critters and the like from getting into my shoes.

In areas where the trail tends to be brushy, or if I'm going to hike off-trail, I like a non-waterproof, breathable gaiter like the OR Rocky Mountain High for brush protection since I really dislike hiking in long pants.