I've used GROM in a few cars now and have found them very good and easy to install. The last one I did was in my E46 Touring which was dead easy as it just plugged in in the boot instead of the CD changer - basically open a flap, unplug one cable and plug in the GROM and all done.
For my 2007 Z4C I am planning to do the same. Does anyone know where the best (easiest access) connection is - connect to where the CD Changer would be if I had one, or into the back of the radio unit itself ?