If you are one of the computer noobs, who click install without even reading anything, and expecting the computer to do the rest, this is simply not for you.
There are loads of codecs, but its up to you to decide which ones are necessary. If you do it wrong(or try to install it all) you will end up with a ruined OS, and a media player working with less file types than before.
If you dont know what you are doing, it has options like "minimalist" "just play" "lite edition" \"player edition\" in it. Either use them or dont even bother downloading it.
However if you have some experience how to configure this, it provides many extra encoding options for adobe premiere, 3ds max or similar programs. It doesnt cause any problems, I installed it 6 years ago, and still havent formatted. Since then, I installed it on another pc, 4 laptops and 2 notebooks, no problems at all.
This PACK deserves so much better rating than it gets, as the problem is not the codec pack, but people with zero knowledge trying to act like experienced users. This is a codec PACK, not a miracle worker.