Legendary rocker Paul McCartney entertained fans -- the lucky ones to nab tickets from Sirius XM Radio (Nasdaq: SIRI) and perhaps millions more through the live broadcasts on Sirius and XM -- for two hours last night.

Digging into his deep catalog of solo and Beatles' hits, the show was Sirius XM's way of celebrating hitting the 20-million subscriber milestone.

One can also say that it also marks the end of many of the question marks surrounding Sirius XM.

  • There are no more questions about Howard Stern. The company's highest-paid talent signed up for a new five-year deal last week. There's also a mobile streaming component to the new deal.
  • There are longer any doubts about which way its user base is trending. Sirius XM is just weeks away from completing its sixth consecutive quarter of sequential subscriber growth. It's a safer bet that Sirius XM will hit 25 million subs in a couple of years -- and not fall back to 15 million.
  • There aren't any debt repayment milestones to worry about in the near term. Sirius XM has been able to refinance most of its immediate debt burden -- and at lower interest rates, to boot.

Sirius is no longer buzzard bait. It's consistently profitable, much to the delight of Liberty Capital (Nasdaq: LCAPA) which opportunistically swooped in when things appeared dire to grab a 40% preferred share stake.

There is also no longer a need to engage in a reverse stock split to satisfy NASDAQ OMX Group's (Nasdaq: NDAQ) minimum listing requirements. The beefy 6.4 billion shares outstanding will keep the stock trading in the single digits, but now a split is simply a matter of aesthetics or institutional investor appeal.

Can it be? Will Sirius XM be, dare I say, boring, to watch next year?

I doubt it. There may certainly be less volatility in store for investors in 2011, but this is still a company with room to bump its guidance higher along the way. The resolution of uncertainty may have been the biggest catalyst for serious stock gains in 2010, but there is still the modelwidening potential of Sirius XM 2.0 to watch out for next year. Now that Stern's channels will stream through Sirius XM's mobile apps, there may also be some incremental business gained through its premium streaming for Apple (Nasdaq: AAPL), Research in Motion (Nasdaq: RIMM), and Android smartphones.

The stock more than doubled in 2010. Investors shouldn't expect an encore performance in 2011. Justifying an enterprise value of $12 billion today takes a certain leap of faith. Trying to validate an enterprise price tag of $21 billion (that's nearly $18 billion in market cap and another $3 billion in long-term debt) is more along the lines of a blindfolded parachute jump.

However, just as rightfully predicted that Sirius XM will gain ground in 2010, I still believe that the Sirius XM story has some exciting chapters in store.

McCartney may have left the stage, but now it's time for CEO Mel Karmazin to jam.

Will Sirius XM go higher or lower in 2011? Share your thoughts in the comment box below.

Nasdaq OMX Group is a Motley Fool Inside Value pick. Apple is a Motley Fool Stock Advisor choice. Motley Fool Options has recommended writing covered calls on Nasdaq OMX Group. The Fool owns shares of Apple. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors.

Longtime Fool contributor Rick Munarriz is a subscriber to both Sirius and XM. He does not own shares in any of the companies in this story. He is also a member of the Rule Breakers analytical team, seeking out the next great growth stock early in its defiance. The Fool has a disclosure policy.