Mod Edit: Minor Spoilers mentioned.
I'm just going to say this straight away: These kind of animes bring out the silly little girl in me. And as I'm a 19-year old guy, it's kind of an unexpected thing, even for me. And I mean this in the most positive way possible.
"K-On!" follows the story of Hirasawa Yui, a girl who lives a life similar to your average person on the net, except for the internet part. It's difficult to explain without using the words "carefree" and "lazy". In a way, she probably reflects a majority of people I know, including myself, except for the fact she doesn't really have any real life issues other than school. This changes however when she decides to join the Light Music Club, where she now has to learn and play guitar. We also got 3 other characters that will be focused on: Ritsu (drummer), Mio (bass), Tsumugi (keyboard), followed by another one half-way through the first season, namely Azusa (guitar). Inbetween their "goal" to reach fame, they have a few concerts and a lot of fun. Quite lovely.
The story itself is memorable, but unoriginal and consisting mostly of filler to build character. However, I myself don't consider this to be a bad thing at all. With animes like this one, filler is what pretty much drives it, and I love every single minute of it. Each episode being just as silly, and if not even sillier than the other. Although it doesn't beat Lucky Star, but if you've seen that, you know that's a difficult thing to do.
The music itself is great and quite catchy, but I'm not a big fan of Yui's singing. I know this might piss some people off, but hey, it's my opinion, and I prefer Mio on the mic anytime. The first ED is in this case also my favorite song in the anime, "Don't Say Lazy". The instrumentals are great, although not that original, but still enjoyable.
The art is cute and silly, and like I said before, combined with cute voices I often find myself randomly squeeing simply because of how cute it is. If it didn't have that factor, I would be far less interested in this anime. There are animes for all kinds of people, be it sci-fi, action, romance, drama, adventure, etc. But for me, comedies are my thing because they distract me from how stupid life can be from time to time.
The characters in this anime are a lot of fun, and very different in personality. We got Yui who, as we know, is carefree and quite lazy, and tends to be awkward on the weirdest of times, but in a cute way. Then, we have Mio, who's pretty much the opposite of Yui, shy and easily scared. Ritsu is energetic and a bit of a devil when it comes to pranks, especially aimed at Mio as they are childhood friends. Tsumugi, who's not as energetic as Ritsu, but still enjoys the little things in life, and she's also the "rich kid" as you may call her, but thankfully you won't notice that in this anime. And later on we get Azusa, a very serious junior girl, but also easily distracted and really silly at times.
All the characters share one thing: They're happy high school students who like to play music. The bond they have is emphasized in the anime, as each episode tries not to leave anyone out. And once again, that's one of the things I love about it.
"K-ON!" is not for everyone, but if you like cute and silly characters with a cute and silly story, then I definitely recommend giving it a shot. Just don't expect any real seriousness, except for when they prepare for concerts. For me, it's a proper balance of a funny story, with funny characters and fun music. It's definitely not original, but as long as it's executed properly, it doesn't have to be, as long as it's fun. And yeah, like I said pretty much a million times... This anime sure is!