In my opinion K-On! is one of the better shows in 2009. And yes, it is better than the Haruhi Season 2 (sorry, Haruhi fans!). I think K-On! is one of those anime that would move people, getting people in making music or at least looking up the rockstars, popstars, artists who are ever mentioned in the series.
Nobody can deny that K-On! isn't a mega success. I doubt it's only the otakus buying the blu-ray of K-On!. I think K-On! can attract all kind of audience, from boy to girls, from young to old; the plot is also not that sophisticated that nobody can understand/comprehend. It all about making music with your friends can be that fun.
Also I didn't watch K-On because it's made by Kyoani; it's more because it's quite well made, the plot of this anime is good enough, the comedy is just right, the pacing is really good, all the references to different artists are just awesome; additionally, the creators of this anime has done a really nice job by inserting several good songs performed by the main characters of the anime (the
singles and
album do sell very well in Japan).
I give K-On! a 9 out of 10!