I like neoprene protectors, such as this one from Lizard Skins, for two reasons. One is that they're easy to take off and clean, but the main reason is I get quite distressed by the sound of the chain crashing on to the chain stay as I ride over bumps, and this stops that completely. I've felt it more since I've had a carbon bike, which makes a sort of hollow cracking noise, but I have one on my mountain al bike as well.