What can i say? K-on is too shallow.
Here's my quick review (contain no spoilers)

Pro:
   +Moe girls: from big boobs to small boobs, from forehead(yes, there's fetish for those, some even want SCARRED forehead) to armpit, from student council to imouto, from swimsuit to sailor uniform, got to have 'em all!!!
   +Decent music: (yes, decent, not THAT good, no where near the quality of God Knows, Lost my Music, etc) Don't say 'lazy' is quite good though.
   +Some light laughable original humor.
Con:
   -Full of fanservice: the whole thing is fanservices.
   -Shallow story: there's no depth to this one at all, you can pretty much predict what gonna happen next easily if you 've watched a few slice of life anime/manga. if it is for you then dude, you need to read/watch more stuffs, not only anime and manga.
   > this one is depend on people ( i guess )
   -Poor character designs: the mangaka can draw cute girls, yes, but almost none of the girls' personality is unique at all. Yui is stupid, TOO stupid perhaps, Ritsu is background girl type, with normal sexual harassment jokes( face it, she has almost no highlight points at all), Mugi: no, just no, one messed up character. Azusa+Mio: ok face it. they are mixtures of everything 2ch finds cute: Long hair, tsundere, blushing, easy scared = crying, etc etc etc, can't list em all. the rest? you can figure by yourself

say, if you can prove me wrong, then go ahead and shoot me. ( prove me wrong first, THEN shoot me)