Here's a summary of my experience so far with slacker and xm.
Slacker: I signed up for slacker premium in an attempt to be able to play any artist and their full albums. This does not work on the model s for some reason. Seems like slacker is not fully implemented within the car itself. However, if you pair your phone via Bluetooth and use the slacker mobile app, you can just use your phone to use the complete service.
When I realized the car wouldn't work in premium mode, I found out that the car automatically logs in with the tesla free account if you log out of your personal account and restart.
XM: I was a Sirius user in my old car and a Howard Stern listener. You can't simply add the model s to your account or switch radios from old to new. They make you cancel your Sirius account and apply the balance to a new xm account. This would be fine except that on Sirius the cost for channels that include Howard stern costs 14 dollars per month and the cost for xm is 18 dollars per month. They don't offer the same packages even though they are the same company now. If this bugs you as it does me, the work around is to simply use their Internet plan and once again use your phone on Bluetooth to listen for less money. Plus you can pause the live radio, or listen on demand across all your devices, like computer,iPad,or even your home stereo if it is Internet equipped.
I think it's a bit of a shame when your phone is a better gateway to slacker and xm than the car itself but in reality, the car is just using a 3G connection(for Internet), same as your phone or worse. It would be nice if the phone's controls would come up on the big screen. I guess maybe that is what you pay extra for with xm.
I think slacker needs to be updated to be fully functional on the car. That would at least make slacker worthwhile.
Would love to hear others experiences.