Pros:

K-on!
It actually works.
Removes the old boring voices, and adds better cuter ones :'D

Cons:

None.

Conclusion:

Really Loved it! :D


Solution Here Again so it wont get lost:
Its a nice mod and since many people are having issues getting it to run i will post what i did:

1) of course donwload the file first
2) get GFCscape (the first entry on google(nem´s tools))
3) save the voice folder in C:\Program Files (x86)\Steam\SteamApps\common\left 4 dead 2\left4dead2\sound\player\survivor
4) open the l4d2 anime voice.vpk with gfcscape(dont put it in your addons folder!!!)
5) Delete the voice folder
6) drag and drop the voice folder out of gfcscape
7) in steam go to your games and rightklick left 4 dead 2
8) click on properties and then select the option above creating a desktop icon(start optionen in germen...suppose its starting options in english)
9) type -console in the window that pops up
10) select ok, close the properties tab and launch the game
11) in the options under mouse and keyboard select developer console enable
12) close the game and reopen it. now a console should appear. type snd_rebuildaudiocache into the console
13) a loading screen should pop up and take some time till the loading bar ist filled. when it is the console should pop up again. when it does type exit into the console
14) try to lauch a game(for testing a singleplayer game is suited best) and press z. now choose some of the options and check if you have normal sounding anime voices(from the K-ON! characters) or if the voices are very slow or simply non existent if that is the case check 15)
15) make sure that you didn´t drop the l4d2 anime voice.vpk file into your addon folder. in either case redo step 12) to 14) untill succes.

I had the file in my addon folder and the voices were very slow. 

I had to do everything from step 12) twice, but now it works perfectly. NICE MOD!!!

ps. enable the subtitles, so you know it when others give you a signal or are simply reloading...cause "yay" is not very informative XD


Note : i tried to post this as a comment but i would always get the "no urls in comments" error...

if i have any mistakes in my checklist please feel free to comment