Yep! I gotta agree with Funky Bunnies, you cant beat a base mesh out of Maya, Max etc before getting into zbrush.

Again this is part of my actual workflow. Maya for base mesh and uvs, then off to Zbrush (if need be) for extra sculpting for displacemnts or normal mapping and sometimes full textures. then back to maya. To build and retopologies is one step too far in the modeling process as it can be a long haul without making it drag on.

Also I find Zb very useful for pushing facial expressions even further as you can 'grab' a handfull of verts and move them in one go instead of the one at a time in maya. Again, my base expressions are done first in Maya to make sure they are okay.

Another great thing with all of this is that once you have a base head built that you are really happy with you can use it over and over and sculpt different heads in zbrush without rebuilding your head every time. Of course there will come a point when your skills will have improved and you'll want to though...

cheers
J