before i go any further, let me  clarify that i am grateful that koz has provided the community with such great hair for as long as i can remember. theyve always been free, even though his hair is/was often far better than what was offered in the  marketplaces and he could have made a killing as a merchant. i used to use his hair exclusively because it was so much better than anything else out there. that said, ill tell you why i personally stopped using it:

koz's hair is very nice, but its also very static.
i used to like it a lot - its the only thing i would use. but the problem is that with the exception of the 'messy hair' theyre like a helmet of hair which is made even more static by the fact that they have very strong baked-in highlights.
these days my favorite hair  model is by far the sappphire fox hair from daz, with the 'in love again' poses/textures by eowyn/firebirdz. its a nice quick, dynamic looking  base which rio or myself can touch up with a bit of postwork.
either that, or just handpainting the hair entirely. hopefully poser7 will have a more refined hair room, something along  the lines of sasquatch. as it stands, IMO, the only thing holding poser back is the hair room and the character rigging system. get those two up to par and the average  level of poser realism will go through the roof.

cheers,
-gabriel