What you have here is a fairly typical 4-koma manga about a group of high school girls in a music club and all the resulting shenanigans and fun that they have throughout the years. There's no serious story to be found here, no emotional moments, no deep message-- just simple comedy and cute girls interacting with each other. It's what you would typically expect from a gag manga focused around young girls. That might be a good thing for you and it could also be hellspawn depending on your tastes and preferences. Don't read K-ON if you want something more than that; it's very simple.
The art is decent enough. It starts out pretty simple and then evolves over the course of the series. The characters you see in chapter one are going to look completely different from how they look in a 2012 chapter. Most of this can be attributed to KyoAni's popular anime adaptation and Kakifly deciding to emulate and portray that a bit more in his work. They look cute and the art gets the job done, so there's not much to complain about or mention here.
Jokes, story-- it's all very average. You aren't going to be laughing to tears every chapter and you aren't going to be super invested in what's going on. If you are coming in from the anime and expecting the manga to be around the same quality, you will be sorely disappointed. KyoAni's adaptation is much improved from the source material and added in many scenes, character development, dialogue, and most importantly, music. It turned an average 4-koma into something amazing. You will not get that level of quality here so you will need to throw away any preconceptions before reading if you want to have any hope of enjoying it.
For the first three years of high school and until the time when the original four girls graduate, it's an above average manga and certainly enjoyable. And cute-- there's little denying that. If you liked the anime then you will definitely enjoy the main portion of the manga.
The new manga that continues after the anime, however, is quite bad.
Why? Because it is being stretched out and continued needlessly after a very satisfying ending that did not need any sort of continuation. In a way, continuing after high school like this throws away any sort of feeling or tone that it had going for the first 52 chapters. Kakifly is trying very hard to emulate that same tone with the college portions but it's clearly not working. The last chapters feel almost like they are something else entirely-- at times like it's not even K-ON any more. It just doesn't work with the characters split up and in a completely different environment.
There's not just one new character introduced, not two, but FIVE of them, splitting the focus between the girls even further to the point where nobody is able to get a significant amount of time or development any more. All of the new characters save for Akira are extremely bland, and in the latest chapters, it even feels like Kakifly regrets putting them in. There was really no need to add this many new characters in. It's very contrived and seems like Kakifly's way of trying to freshen things up and make it all different which is not at all what the fans want and it's clearly not something that he was ever capable of doing well. Sumire, one of the new girls in Azusa's side of the story, is derivative and almost a complete clone of Mugi, even filling largely the same role as her, looking the same, having the same rich and high social status, and even being revealed as being childhood friends with each other. It's not just Sumire, either, but all the other new characters seem like a blend between one or two of the main characters' more prominent traits. It's sad, but Kakifly is showing here that he is running out of ideas for new characters. He can only create them based on one superficial gimmick and then play on that the entire time.
For Azusa's side of the story, she is the only interesting or unique character there, which is a real shame. The other four are dull and bland with nothing exciting about them and it makes her entire portion of the manga a bit painful to read at times. I love Azusa more than anything, but even seeing her act cute and watching her struggle as the new leader of the club— growing and growing, being a well-rounded and developed character— isn't enough to make me enjoy reading her chapters. The other four characters and the stale, repetitive situations that they go through each chapter detract from it. There isn't anything fun about watching the girls go on a beach trip for a third time.
Kakifly is great at making cute panels and amusing jokes that will make you smile and 'hnggg', but he can't do a proper story all that well. The new manga is very, very weak, and if it was going to be of this quality then I don't think it should ever have been continued in the first place. It ended nicely and now it is simply being milked and continued to a point where even the most dedicated fans are growing tired.
Anyway, that's enough ranting now.
As long as you aren't expecting something life-changing and amazing from the manga it should still be worth reading, especially if you are already a fan of the series. It's pretty weak, but not weak enough for me to stop reading. It's mediocre (especially in the writing department), but it's still capable of being enjoyed.
If you're a newcomer to the series and want a nice, relaxing slice of life or comedy, please watch the anime instead even if you aren't typically an anime person. It's much, much better. KyoAni did a fantastic job with the series.
