I don't tend to focus on the oem modules as in every aspect for me, they're inferior in both features and performance to current high quality aftermarket electronics - hence perhaps those who want the oem stuff having to research and research some more, then perhaps try various things and be prepared to research more and then research again if things don't quite work etc with their particular E39 !
I'd also hazard a guess that perhaps in the next few years, the latest Bluetooth specification which would then be used in forthcoming mobile phones will not work with the oem modules for the E39 (E46, E53, and E38 amongst others) era cars. Wouldn't be so 'much' of an issue if the modules were perhaps $30-50 or so. However, as they cost so much, for me the risk is too high to spend it on old, non warranted electronics given that the next ranges of higher end smartphones etc might not work with them!
I understand though, that many owners out there want to keep oem audio/modules in their cars, so it's a case of research, research, hope, try and 'hopefully' things will work !