There are a couple of issues, with many CODEC "packs."
First, most will replace good, existing commercial CODEC's with ones that are hacked, or reverse-engineered. Some of those do not work, or do not work well. That is why Adobe "hides" its installed MainConcept CODEC's, by using a non-standard naming convention. However, as the CODEC packs also rewrite the Registry, the priority of the commercial CODEC's can be altered.
Next, many CODEC packs also load other components, besides just CODEC's. One of those is often FFDShow. That particular one can cause severe problems with Adobe programs. It can also be very tough to fully uninstall. I hear that many CODEC packs, that do install that, and other components, do a better job with the uninstall function, but with some older versions, FFDShow could not be fully uninstalled. That is what Harm was warning about - things might be better now, but the Adobe Premiere Forums are full of horror stories, especially related to FFDShow. Just do a Search in this, and the CS 4 & Earlier Forums for "FFDShow," and read.
I am with Fuzzy (Reply 15), on ONLY installing the specific CODEC's required, when they are required. I also try to go to the source, which might be free (like Lagarith, or UT Lossless), or commercial, such as MainConcept. I do use a few Open Source CODEC's, so it is not the "commercial" nature, that is the problem, but the exact CODEC's, themselves. I also do not want any program removing any of my CODEC's (many costing quite a bit of $), or altering their priority in the Registry. I hate spending a day in RegEdit, cleaning up a bunch of "stuff."
Good luck, and glad that K-Lite has performed for you. In enough cases, it has not, for others.
Hunt