Always a tricky subject! Dom B was spot on in his post back in August. The OE manufacturers and their supply base spend a vast amount of time and money ensuring that their products are safe above and beyond their intended use (with respect to environment - loads etc). It is very easy for an aftermarket agent to take one of the OE parts and faithfully reproduce the outline product - that is the cosmetics and the key fitment details (in the Ball Joints case its taper, gauge diameter, thread etc). What can be overlooked - often with dire consequences is the real detail; material conditions (not just the surface hardness), durability capacity, sealability performance etc all of which can have a seriously adverse effect on longevity of a product. Where an item is "safety critical" then ther can be serious repercussions!

My advice: for key compoents / assemblies (inc steering - suspension - transmission - braking etc) stick to the OE part or second best to parts that are manufactured by reputable companies that do supply the same genre of product for an the OE market (although they might not supply that exact same part). It might be a bit more costly in the outset but what value do you put on your life and the life of others.

Sermon over!