While we're not sure how big a slice of the Guitar Hero pie the Wii version has taken, the franchise has generated some serious revenue. In North America alone, the series of games has earned over $1 billion. In fact, the third title even did so well as to be the top title in 2007.
Other data regarding downloaded content is also provided. However, that doesn't really apply to us, as we've been waiting for downloadable content for some time now.