I've had XM for a couple of years, and enjoy it. I mainly listen to music. I've expanded my music horizons listening to channels that I'm not familiar with. Even though they play 100x the variety of what a commercial station plays, there are a limited number of songs available, and after a few months, you start hearing the same things over and over. At least it's not like commercial radio, where you start hearing the same thing every hour.
The type of radio you get greatly impacts your experience. I have a Pioneer Inno, a portable docking radio, so you can put it in multiple docking stations. The thing that is really good about this radio is that it has the ability to pre-record channels, and you can then listen offline. You can skip songs that you don't like, skip irritating DJ chatter (unfortunately some channels have too much of this), and skip commercials if you listen to one of the few channels that has it. This is a feature of the radio, not XM. The other advantage is that you can listen to the music "offline" (not connected to the satellite).
That last point is a major advantage. I don't consider XM to be "portable". Teh radio doesn't work if it's not connected to an antenna. If you are in the middle of a house, it probably won't work. Need to be near an outside wall/window. If you want to move to another room in the house, you need to bring the antenna with you. It's doable, but the PITA factor will prevent you from moving it around in the house, most likely. FM radio is far more portable than XM. You can turn on an FM radio anywhere and receive something. Not the case for satellite radio.