Positives:
-Fully Integrated with CD53
-18 Presets are available on the CD53
-Sound Quality and Clarity is Amazing. Better than you'd expect.
-Pretty fast in picking up channels and acquiring a signal-provided the antenna is mounted properly
-Reception is stellar even with a low budget aftermarket antenna
-Easy to set up in car
-Sirius does not count these older receivers as "factory installed" or "oem car radio" so the limitations and rules do not apply this receiver that prohibits subscription transfers between in car factory installed Sirius radios.
-Works with USA-Spec, DICE, and any other iPod Integration kits.
-Most units can work with both the CD53 and/or OEM Navigation so you have the flexibility of upgrading to nav and not losing Sirius
-Resale value at this time is out of this world. Expect to sell your used units for either what you paid for them or more when you are ready to sell them.
Negatives:
-Scroll features suck on the CD53 and this for me is an
almost deal breaker
-Channel Surfing using the Radio < and > buttons are really slow going between 100+ channels. Because of this Presets are a must and the Steering Wheel's << and >> keys use only preset selections when surfing so that is good.
-These Sirius units
may not get XM Only packages and channels
-Scarcity and absence of production have drove up the prices of used units. I was ready to pay close to $500 for a receiver last Fall and most used 2nd/3rd gen receivers on eBay can bid all the way up to the $900s if they come with subscriptions.
-BMW either does not warranty or service these units at all or you have limited service/warranties on them if you buy a CPO with a receiver installed.
-Fahkra Antennas that this unit utilizes tend to be more expensive and rarer than the normal-connection Sirius antenna.
Here is Sirius on the OEM BMW Navigation Screen as shown in SNA77's 2004 325xi:
Secret Agent Super Dragon
Full Review for the BMW E46/E85/E53/E39 OEM Sirius Satellite Radio Receiver 
Linear Mode
