failing water pumps on BMW around 40k is the norm. My water pump on my e39 fails every 35-40K. A buddy of mine has a M roadster and his fails every 40k.
Buddy of mine has an m5... clock work my friend.. every 40k.
Also, what I have found is that ever 80K or so... full overall of the coolant system needs to be done. I have had many BMW mechanics at reputable independent shops all say the same thing.
This is the nature of this beast... you either live with it or get a Honda
