Hi all. I just wanted to post about upgrading to Slacker Premium since there is a lot of contradicting information out there.
It's easy: Just create a Premium account on the web, and then click the "log in with your own account" button at the bottom of the Streaming page in the car's center console. All customized playlists and favorites will appear immediately. At this time, Slacker Premium is $10/month, billed monthly. The car saves the login credentials, and everything works smoothly.
You can easily create playlists of (almost) any set of songs -- including a complete album. When listening to the playlist in the car, you can use the forward and backward track buttons to listen to the songs in any order and as many times as you like. A slight annoyance is that certain songs (maybe 5% of Slacker) are not able to be added to a playlist due to licensing restrictions. You cannot use the backward track button when listening to a station even with Premium. It only works for playlists.
Premium offers a larger song library and more channels (so it appears to me). When you favorite a channel from the web or mobile Slacker interface, it will be added to the car's favorite list automatically -- no need to log out and back in. Sometimes the update is not instant. It may take a few hours. I always put together my playlists at night in the house, and they are always ready in the morning in the car. Perhaps log out/in would refresh right away if you are in a hurry.
Overall, I wish Tesla allowed the use of Google Play Music in the car, since I was using it before getting a Tesla. However, Slacker Premium is nearly as good, and I will probably switch over completely. I've recently been listening to the "Classic Rock Deep Dive" channel, and really like the song selection a lot as I have with other Slacker channels.