The coolest feature of a recent Pontiac Vibe I tested was its XM satellite radio. There were dozens of non-stop stations in great quality covering a wide array of genres and even a decent selection of talk radio. Today XM's main competitor, Sirius, announced it has hit the 500,000 subscriber mark. That's a huge achievement for this upstart medium that initially had to tread water to survive. Now even Eminem is getting his own Sirius hip-hop station and the company is teaming with the NFL to broadcast all of its games this season. However, Sirius is nowhere close to the 2 million plus subscribers XM has, mainly due to the fact that you can get XM installed in factory stereos of GM products like that Vibe and even Porsches in 2005.