Don't know if any of you have seen the discussion thread over on M3Post regarding the Mr12volt mmi box, but I've been following it for a while and have now fitted one to my own Z4 (as well as 2 other owners cars) and it's a great bit of kit.
It's biggest improvement over the normal MMI boxes is that it connects into the MOST fibre optic network, thus giving you a digital signal instead of an annolouge one via the AUX socket. The integrations looks far smoother and looks more OE as well. I've been running mine for the last two months now and absolutely love it, no hiccups or issues whats-so-ever.
I've made a How-To video for my YouTube channel, here >>
Hope you like it, links to buy them are in the description.