I got a GROM BT3 (which is capable of Bluetooth audio/phone, and with optional adapters aux-in, ipod/iphone/android-in, or HD Radio) for my '06 E53 and am satisfied overall but I had some issues.

Make sure to buy it directly from GROM USB and Android Car Kits | iPod, iPhone, AUX Car Adapters | Bluetooth Hands Free Car Adapters | Bluetooth Car Kits so you will be sure they can confirm you get the right model and harness for your specific system and options you want. That's where I bought mine and I got everything I needed in one package.

Pros: Bluetooth works fine. Connects very quickly, sound is fine (way better than FM modulators). I purchased the AUX-in and it works fine as well. I have factory bluetooth for phone and wanted to keep it as it works very well and am able to have both working without any problems. I was able to install it myself with very basic tools and I don't know anything about car audio.

Cons: A couple times the connection would drop and reconnect and repeat every few seconds. It's annoying so I had to re-pair bluetooth. It's simple but you have to reset the GROM and then re-pair. It's pricey, and the instructions are very lacking. It took me longer than it probably should because it took me forever to figure out the wiring and which wires to splice into - I took my time (a long time) to figure it out before doing anything and eventually got it. For my particular unit it automatically switches from bluetooth to aux, but it won't switch if you are trying to go from bluetooth to aux using the same device as it defaults to bluetooth (which is not a big deal for me since bluetooth works fine I never use the aux-in).