These are a nice replacement when your stock unit wears out. I've bought these for several bikes and they work well. I actually tried a cheaper unit that was metal/plastic and it wore through much sooner than the Acerbis.
The Acerbis Chain Guide Block easily replaces the entire O.E.M. unit and ensures durability and high resistance to shock and wear from the chain. Made of PA6, it is made of two parts that allow installation without removing the chain.