A couple of things based on my experience:
- yes, you can get the car's slacker account. send it as a request with your car's VIN and request the username and password
- some things that you can do with the person slacker account
- log in from a computer/mobile app
- turn on high bitrate sound (not sure I heard a different)
- turn off explicit lyrics (this doesn't work - I turned it off and it still plays explicit lyrics)
- play slacker on your computer/mobile using your car account so you and like/dislike songs. however, you cannot create custom/specific playlists - it is done the same way you do it in the car. pick a song and create a station with it. then thumbs up and down to play more of some songs and less of others