The trial is 4GB of data or 90 days, whichever gets used up first. I used it this afternoon with a couple of iPhones. Works beautifully. I had my iPhone connected to USB for CarPlay and turned ON SharePlay so that the other iPhone could act like the remote controller for Apple Music instead of using the car’s track pad to make selections.
With a press of a button on the steering wheel, I can also bring up Siri and tell it to play anything from Apple Music. If it isn’t downloaded to my iPhone library, it is simply streamed from the Apple Music service over the car’s WiFi. No hesitation, no buffering. It just worked.