I read that most guys prefer buying from www.raxles.com
cause of a better quality product. Part of their marketing is that they only use new joints.
I've heard some people say that rebuilt axles only change the outter and not the inner joints. I've also heard people needing to go through several sets of rebuilt axles (like Master Cylinders) before everything was kosher. Only problem with that is either your labor (if you install yourself) or your down time (bringing it back to the shop and leaving your ride there).