I have had both XM and Sirius satallite radio services. I have a Delphi SkyFi unit with both the home and car kits. I have the SkyFi ported through my Sirius-ready Kenwood deck. I also had the Sirius tuner for the Kenwood since they were running a promo for a free tuner and 30-day free trial. Before the trail ended, I cancelled Sirius and sold the tuner and antenna. I still have my XM service and love the service. I'm thinking about getting a second XM unit since my wife uses the XM at home so much.
Sirius had very audible artifacts in their midrange and treble, especially in the symbols onthe jazz channels. Sirius sounded like a low bitrate MP3. XM doesn't have quite as much low end bass, but the overall sound quality is comparable to a 160kbit or 192 kbit MP3 file. Is it as good as a cd, no, but it's significantly better than radio. Also, Sirius programming played the same 40 songs over and over and there was a significant amount of overlap between various channels. For example, a later Led Zepplin song would be played on about four different channels and a current release RHCP song would be played on five, all within 10 minutes of each other. I also experienced significant signal drop outs with Sirius all over the Midwest.
As far as bringing in XM into your Axim, I'm not aware of a way to do it. You can bring XM into your PC and capture it as a .wav file, then edit the stream into individual songs and convert them to .mp3 or .wma files.
Here's a link for a wealth of additional info:
XMFan