What kind of experiences have you had with them? What are the ones to use & avoid? Which ones replace/demote system codecs, and which just add more?

I do some video encoding once every blue moon, and like to experiment with different formats to see which offers the best compression:PQ ratio. K-lite has served me well for this. However, it seems to give higher merit to some codecs that aren't best for much more mundane tasks, such as decoding MP3s and DVD movies. I've run into some issues in the past.

This is why I'm becoming a big fan of players and encoding tools that use their own internal codecs, instead of making system-wide changes that can cause subtle problems, which are difficult to identify & correct.

But that's me. Anyone here have any particular favorites, and why?

if you know what your doing, you;'d be better off selecting individual codecs and self installing them.

K-Lite worked well for me too, used to be a real pain trying to find out what would play what before that came on the scene.
I don't use it anymore though since I started using VLC to watch videos.

That.

I'm done with codecs. Don't have to install anything in my hard drive, and VLC plays everything I've thrown at it. I'm not doing any encoding though.

Same here.
I can't understand why anyone would bother with anything else.

+1 for VLC

I pretty much only use VLC for everything. I know a few games (especially Bethsoft games) don't react well to some codecs and cause crashes, but so far the basic ones with VLC/Windows seem to work fine.

VLC is pretty much amazing. Use it!

VLC is great, and so is Media Player Classic Home Cinema, which also plays pretty much everything out-of-the-box.
If you want to use a media player which has no built-in codecs, do not install a codec pack. Instead, install the individual codecs for the file formats you would like to be able to play.

VLC for the win, if i want Compression dlls I'll install them manually.

All hte Klite and Nimmo Codec packs are is the Decompressors for people that still use windows media player for everything.

FInd the Compression you want and obtain the Compression and Decompression DLLs from the provider themselves (xVid, DivX, 3vix, WMV-VC1, etc etc).

Codec packs are good except having so many codecs trying to set themselves as the primary will cause issues in windows.