Every week, you get your serving of cuteness and some music. This is the kind of show where you don't have to watch it in high-brain-power mode. You can just chill, turn off your brain almost completely and still get the concept. There are moments in this show that will make you laugh and some that will nearly make you cry.
Iif you are looking for in-depth character development in a series, K-On! will not exactly offer that to you. Ritsu, Mio, Mugi, Yui, and Azusa did have some character development but not to the point where it will satisfy your need (most likely). Also, each character have their own interesting personalities that make the show even more enjoyable.
Although the show is named K-On! (keion for light music), you will not really get the hardcore music scenes and songs that you'd want. If you look at this show as a music club with 5 girls, you will be pretty much disappointed if you take it from that point of view. If you look it at the other way, 5 girls who come together because of music, you will not be disappointed.
In brief, I recommend K-On! if you're looking for a moe anime that will give you the lulz, but don't expect it to be really deep because its based after a 4-koma manga after all.
