XM Satellite Radio was the first satellite radio company to come to market. XM was merged with Sirius in 2008 and the consolidated company is struggling to pay its debt and stay in business.
XM tried to woo shock jock, Howard Stern but ultimate lost out in a bidding war to competing Sirius.
Many enthusiast thought that XM Satellite Radio's content was better than Sirius but today most of the channels have been merged leaving little consumer choice. XM had access to Major League Baseball which was one reason why people picked XM over Sirius. Today that argument is moot.
After the merger with Sirius, XM has become basically one with Sirius although the channels are a bit different for users.
Here's a link to XM's channels: www.xmradio.com.