You've come a long way, Sirius XM Radio (Nasdaq: SIRI).

The satellite radio provider is topping the 20 million subscriber mark this week.

It's an impressive milestone, though it's long overdue. Two years ago, Sirius XM expected to close out 2010 with 22.1 million subscribers. After closing out 2008 with 19 million subscribers, the recessionary downturn took its toll.

As new auto sales tanked, Sirius XM's best source for new accounts also dried up. After back-to-back sequential declines in net subscribers, Sirius XM began to bounce back in the second half of 2009.

We knew that this was going to be the quarter in which Sirius XM would finally hit 20 million subs. It closed out its third quarter with 19.9 million accounts and is publicly targeting 20.1 million by year's end.

It's impressive that Sirius XM was able to hit the mark in November, instead of the post-holiday activations that were common during the platform's early years when retail subscriptions were the key to success.

Gaining a lot of subscribers -- now second only to Comcast (Nasdaq: CMCSA) among premium entertainment services -- is clearly a good thing. Don't let the bottom-line effect in the near term sway you otherwise.

Sirius XM has been able to drive its average acquisition costs down to $59 per subscriber. It obviously isn't making that back during the initial prorated quarter, but the company's low monthly churn rate of 1.9% proves that it's a worthwhile investment over time. If Sirius XM is too successful at nabbing new accounts, it may affect its bottom line during the quarter itself, though obviously this is a good problem to have in the long run.

This isn't the time to take it easy. If Sirius XM loses Howard Stern next month or plans to bump its rates higher next summer, the streak of what will now be six consecutive quarters of sequential subscriber growth will be tested.

There is still no real threat to Sirius XM in the realm of premium radio. The success of Web-based Pandora Music and Apple's (Nasdaq: AAPL) iTunes hasn't really slowed Sirius XM down.

Sirius XM points out that it took longer to land its first 10 million subscribers than its next 10 million. Does this mean that it will have an even quicker path to 30 million, or will the company run into resistance as it tries to reach out beyond its core of active commuters?

Enjoy the milestone, Sirius XM. Just make sure you continue to drive forward.

When will Sirius XM hit 30 million subscribers? Share your thoughts in the comment box below.

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Longtime Fool contributor Rick Munarriz is a subscriber to both Sirius and XM. He does not own shares in any of the stocks in this article. 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.