I don't have any personal experience with them as mine was replaced under warranty due to leaking seals around 80k (or so the previous owner tells me).
I've heard of members buying them used for around $100-150. As long as you're buying from a reputable source, I would assume manufacturer ones are ok. They take ones that need repaired and completely take them apart, replacing seals and whatnot.
Have any links to the ones you're looking at? We can maybe help better that way.