For me it is not so much what as how. Being hearing impaired, audio is not that important. Have a lot of CDs but am slowly transforming CDs and Vinyl to mp3s.
For a long time the problem was that mp3 players (and can just plug a flash drive into the armrest) had no indexing. For awhile I was just buying cheap bulk 32 & 64M flash drives and putting a single album on 32 and a double on a 64.
Now with Android ICS I can index the albums and select individual songs by title. All fit easily (dunno the capacity) on a 16 GB SD card (ten bucks) and each shows as folders which I can select or singles from the folders into a playbook. Anything more I can download using a 3G connection (have a mobile MiFi device that can accomodate up to five WiFi connections).
Nice thing is that modern tablets can multitask so can play music (and quality is far better than Sirus IMNSHO) and display the Sat-Nav, read eMail (while stopped), surf, display engine diagnostics, or make a handsfree phone call (music holds) all usiing the same device. And is an easy upgrade when new tablets come out since the form factor is standard.
Only piece I am missing (any ideas ?) is a composite video input so that I can incorporate the back up camera and switch automagically when in reverse (or want a close look at the car behind). Right now that takes a seperate monitor.
Then I can look at replacing the radio in the dash with a MFD (multi function device) you can take with you when not in the car.
Not perfect yet, local backup and recovery for the applications is an issue and ICS has some BlueTooth problems that need correction but expect RSN (been hammering on the vendor for a month, finally admitted there is a problem).
Everything is here now, it is just a matter of integration. Am sure CJF is working on something similar but my engine instrumentation also can display and clear diagnostics, something am sure they do not want mere owners doing.
BTW, this is a kludge but both cupholders are usable.
Why settle for just audio ?