Well if there was any doubt...

that K-On! is popular in Japan...

http://en.akibablog.net/archives/2009/04/50830031.html

Fender Japan ordered 2x the yearly production of the '62 Jazz Bass the character uses
since they are selling out so fast. The $780 headphones in one episode are selling out as well.

So far I've seen the OST and character singles, Mio's CD seems to be the better of the two (Yui and Mio) so far. Her VA has a much stronger voice, and I wouldn't be surprised to see her move on to a more professional career after this if they aren't knee deep in K-On! Second Act.

Yeah, it has been spreading for months.
Still, I have a problem to catch on fire, although, I think an all (unrealistic) girl band is quite interesting....but then again the japanese have those fetishes towards female idols.

Which is interesting in and of itself, since the female population has been held in such low regard
for several hundred generations now... Granted there was some level of respect as a homemaker
depending on which part of the culture you're looking at, or purity perhaps.

To me I don't see a all girls band unusual. I could point out at least a couple all female groups.
BOND, Dixie Chicks...etc

It's cute, it's fluffy, it's slice of life. The music isn't entirely grating on the nerves.
The moe is a bit much, but I've seen girls that act just like the characters... so it's not out of the realm of believability. It rather relates to my own situation trying to find an instrument to play right now.

So it's a pleasant distraction.