I kinda forgot about this thread; probably overdue for an update!
I did pull the trigger and bought the box for $40. Connection was pretty simple; one of the connectors was slightly different than the matching one on the CD changer harness in the car, and required a little more force to connect it than I liked, but it went together just fine, and worked right out of the gate. Score!
The brains of the unit stays in the back cubby where the CD changer ordinarily resides, and has a really long cable that is meant to stretch to the dash or console for connecting an iPod; I didn't pull it all the way through, but did get it so that I had it close to the driver's seat for testing on an old iPod I have. Once I verified it worked ok, I picked up a Bluetooth receiver/adapter that plugs in in place of the iPod, and was then able to connect my iPhone via Bluetooth. Double score!
It works pretty slick too; when there's an iPod connected to it (either physically or via Bluetooth) clicking through the options with the Mode button brings up an extra option that shows CD-01 on the display. Once you're there, you can use the buttons on the wheel to jump forward or backward in the selected playlist. It doesn't show the song title or any other info on the radio display, but it is smart enough to pause the song that's playing when you shut off the ignition.
Keep in mind that this unit is very likely several years old; I'm sure that the more recent products from BavSound are more feature-rich, and ordering one specifically for the e53 will result in a better fit. I'm just happy that I was able to add Bluetooth sound to my X5 for less than $60.
