Next thing I did was the audio system. I was pretty disappointed with the stock base audio system.
I had a bunch of equipment from my previous car, so I decided to use as much of that as possible.
Although many suggest keeping the OEM HU, I decided to install my Pioneer double DIN HU in order to get a bunch of new functionality: iPod, Navigation, Bluetooth (phone & audio), video playback, etc. Trying to add some of this functionality with the OEM HU was quite expensive, plus I'd still have to recode the unit to HiFi or add an EQ flattening device.
A detailed post on my audio build can be found here:
http://www.e90post.com/forums/showthread.php?t=844394
I installed my Pioneer HU, and a couple of Punch amps I had lying around. Even with the OEM base audio speakers, it sounds better, and I'll work on upgrading those speakers down the line. I've installed a 15" ported box in the trunk for now, but am waiting to get a custom sub enclosure built in the floor of my trunk.
Here are some pics of the audio work:
Before
After