It's hard to put into words what I think about K-On overall, there are aspects that I like but a big issue that I have...
I should explain the plot though, the story follows four high school girls Yui Hirasawa, Mio Akiyama, Tsumugi Kotobuki and Ritsu Tainaka with Azusa Nakano joining in around half way through the series (you can thank Wikipedia for the names) as they keep the light music club alive, including song writing, a few gigs, ect. The characters themselves are ok, their personalities and the way they behave are consistent but the big issue I have with the series is one thing: THE PERSONALITIES BELONG TO THOSE OF (Middle school???) (this is a Australian talking where the educational system is literally Primary School (Prep (5 years old) to year 6 (11-12 years old) and then High school which is essentially Middle school and High school in other places combined, from there its straight to University). I guess it should make sense in that regard but because K-On loves to shove it down your throat that it's set in Japan, it doesn't make a lot of sense to me.
The story itself is mediocre at best. The day to day episodes are ok but there are a lot of gaps in a year. When they managed to fit two years in 13 episodes and then have enough material for a second, larger season, it comes off as a bit lazy to me. I get that their High school students but a Year is a long time...
Because the plot is based around the idea of belonging to the "Light Music Club", there are a lot of singing in the show. I watched the dub and while the non singing parts were ok, I was grateful that the dub kept the Japanese songs and subbed them. The songs have a nice melody to them but shouldn't be herd or read (if you want a good example, listen to the Lucky Star theme in Japanese and then English).
You'd have to be a specific target demographic to like something like K-On, it might be because I don't watch a lot of High School anime (this being my 3rd ever), it didn't keep my interest very well. I've started watching the second season to see if they improve but my hopes are low. If you like High School anime, then by all means watch it if you haven't already, but otherwise just skip it.