There are tons of codec packs on the market right now - Nimo, Gordian Knot, Ace, K-lite, All-in-1, SLD and Kristal are the current favorites. Some have everything (like Nimo) while others are a little sparce (like Kristal). A question I always get is "which one should I install on my computer?"
Therein lays the problem with all these codec packs. Do they do what they are intended to do -or- Are they all fairly equal? The answer is undoubtebly -- NO.
Take the Ace Mega Codec Pack - what does it have. Just about everything from God, the kitchen sink and the hoogie I are for lunch last week. The question shouldn't be what does it have, but how does it handle what it has. And that's where most codec packs fail - they often have incompatible codecs, don't warn you of over-writes or duplicates of known working codecs, and the big one - they do not properly register their codec information in the Windows registry.
Of all the codec packs I've been involved with (either used, or had others use that I've ended up repairing), the Kristal Codec Pack and the Gordian Knot Codec Pack are the only packs currently available that properly registers all their codecs, don't use 'hacked' or 'modified' versions of known, working codecs, and makes you aware of possible duplicates during the installation process. For me, this is so much more important than what is actually in the codec pack. But for 99% of the people surfing the web, the Kristal Pack or the Gordian Knot Pack has everything they'd need. For the rest, keep your WMP9 codecs updated, download a huffyuv codec if you intend on conversion or encoding, and get the Morgan Stream Switcher plug-in if you watch movies with multiple streams encoded.