The problem with replacement parts is they are done cheaper!! The price you pay at BMW is a guarantee on the part as well as the massive margin the make!! but i have had bad parts from them, i had a TPS sensor that didnt work!!!
The companies such as euros, Camberly etc etc dont make the parts but buy in replicas, I have had used all of them and at one point or another you get the faulty part as it is made in India etc and quality control is about as good as a life jacket in the dead sea!!!!
My advice mate, everytime you buy something from a motor factors, take the part out of the box and just do a visual check you can normally spot something array if it dont look right ask for another to compare, if your not getting trade then use the facilties, if you are be as nice as possible!!!