As an XM subscriber of several years, I have to say the sound quality has been disappointing, particularly for home use. When you're out and about, either in your vehicle or otherwise, there really aren't many options. But if you car has a decent stereo system you will definitely hear the difference between a good CD and XM.. XM sounds flat and lifeless in comparison.
For home use I just decommissioned my XM tuner in favor of Internet streaming audio. Hi-res audio streams are available from
www.tuner2.com,
www.shoutcast.com, and other sources. aacPlus is a fantastic sounding algorithm. High bit rate MP3s sound far better than XM, and there are quite a few Internet radio stations streaming MP3 at 128Kbps and above (a few at 320Kbps!). In fact, the hi-res streams put my CD player (a Rega Planet) and CD collection to shame.
Get yourself the free Winamp software! Get a hi-fi sound card! Use a notebook computer like me that won't accept a sound card? Then get the M-Audio Audiophile USB external sound card. There are thousands of streams out there waiting to be listened to!
FWIW, XM Radio does have a Windows-based player, and you can up the bit rate from 32K to 64K which makes a big difference. Two problems though: 1) The player times out and stops playing every hour or so due to "inactivity", even when you're busy at the keyboard; and 2) HughesNet satellite Internet service (my ISP) seems to block the port for XM's Windows player.