I think xm has been around for a few years now but only recently has become more popular among people. As the time it was launched there was a feeling it was going to fail but seems to have taken off since. It is primarily a Satellite Radio service - commercial free supposedly - as everything over here is usually peppered with commercials/ads. You can see some cars driving around with the xm radio antennas – I understand you can even get BBC world service radio on it. I think Garmin is also now partnering with others to make GPS more attractive – for example they are now offering some car GPS models equipped with RDS and partnering with a network of FM broadcasters to provide traffic info. So I guess the features on the 396 is a result of Garmin partnering with xm for this additional service.
As for magazines – I subscribe to AOPA’s Flight Training which is quite well written and has a bunch of technical articles which may be of use to those learning to fly in spite of the differences in the FAA/JAA system.