I did a custom stereo install on my E90. New HU (with Nav), different speakers, different amps, different porcessor. In fact none of the original stereo components are left.
you can find pictures here
http://www.e90post.com/forums/showthread.php?t=2894
The frame is by BMW. It part of the (europe only) radio-navigation retrofit pack.
Please note, that my car is euro spec. So despite beeing a 330 I could order it with a very basic radio and speaker setup wich made retrofitting other parts a lot easier. With the US spec hifi-system (on a 325) or the Logic 7 (on a 330), things get really complicated. For example you cannot combine any component of the Logic 7 with an aftermarket headunit. If you want to swap the hu you also loose the complete Logic 7 processor and amps because they´re fed and controlled by a digital Bus that aftemarket units cannot provide.
Even with my simple setup retrofitting was quite difficult. For example many connections you´ll need (such as a constant 12volts line for the headunit) are not prewired. There isn´t even an ignition lead in the power supply plug for the hu. The factory radio gets ignition signal through the CAN-Bus.
Retrofitting aftermarket stereo components in the E90 is really not for the faint at heart.