I use K-Lite with WMP as the default player (I find the Classic player ugly). Never really had any problems playing anything on it, save .flv files. But that's probably because I use WMP as the default player.
I guess I'm the only one that had problems with CCCP. When I started using Windows 7 as a media player, I really wanted to use the Media Centre application because I have a pretty cool remote controller for it. The main problem was getting .mkv working with subtitles (ANIME!!) without too much hassle.
Long story short, I got everything working using Shark007 codecs. (http://shark007.net/) Give it a shot, it supports 64-bit even in some cases!
I personally use the K-Lite Mega Codec pack with MPC with DXVA enabled and the DivX 7 H.264 decoder as a back up. CoreAVC doesn't seem to like to work on my computer as well as the DivX 7 decoder, but at least it's free and the performance is supposedly just as good.
My brother had decided to use CCCP with Zoom Player and the DivX H.264 decoder just to be different. It doesn't seem to have a problem working either.
I totally recommend trying out the DivX H.264 decoder even if you have CCCP or K-Lite. It's way better than FFDshow's decoder at least.