According to an article by USATODAY Modern Warfare 2 is doing really well. Like we needed USA Today to tell us that :) According to the article:

Based on the initial sales figures for Modern Warfare 2, it's safe to say the Call of Duty franchise has done quite well.

In fact, Activision recently revealed the first-person shooter series has surpassed $3 billion in retail sales worldwide.

...

Earlier this month, the publisher announced the latest chapter in the series, Modern Warfare 2, set a five-day global sales record, grossing $550 million.

It's amazing how well MW2 is doing. From experience I can tell you that the game and the series is a ton of fun. According to Raptr (A new and upcoming IM and game tracking client):

Modern Warfare 2 has broken 200,000 hours and passed Borderlands in 5 days http://raptr.com/search/games

Source

What do you guys think? Are you surprised?